Devaluation isn't really a problem.
You just magic up something else to sell.
Obviously. But the point of the OP was one big spell, singular.
Or a burglary attempt. 
It might be interesting to note the nature of the covenant my mage will be a part of if the game ever gets off the ground.
Basically so we can have maximum story potential we are going to have a regio with a high aura located in the Levant tribunal somewhere between Acre and Jerusalem. However this regio will also be connected to an abandoned wizards tower in Scotland and a townhouse in Venice courtesy of two portal boons.
We’ll start out as a spring covenant with three or four minor income sources. Farming and/or shepherding in Scotland, Glass work in Venice, and trade in commodities between the three locations. Of course the last one can and should turn into a major source with some work.
I hit upon the CrAn spell while trying to figure out what commodities could easily be traded between the three areas. (It turns out Scotland was a major source of freshwater pearls for centuries.)
My mage is not the type to spend the whole yield all in one place even if that where possible. However careful trading of say less the 40 mythic pounds worth per year divided between the three regions would probably never raise any suspicion or cause much instability in the pearl markets. I’ll probably have the covenant recruit (read create one myself) a jeweler’s guild member as a grog, and I know the player who loaned me City and Guild was planning on making either a merchant adventurer or capo as a companion.
As a purely roleplaying choice, my character will probably be quite surprised by the yield of the spell if and when he first casts it. He will probably be expecting to fill a small chest and get quite a surprise. Again he may never tell anyone exactly how many pearls he has hidden away in his lab. Maybe I’ll hide them with a MuAn ring duration circle target.
I fully expect this spell to generate many stories for our troupe
I like the image of the magus getting far more pearls than expected.
Incidentally, Richard Love is right that making money is very easy for magi, and that's deliberate. Earlier editions tried to make it harder, but then I read up on the medieval English economy, realised how much money magi could make with CrAn to make wool, and that a rule fix to make wool hard to produce just wouldn't fly. There are simply too many ways for magi to make money very easily, so it's much better to structure the game around different stories.
My point:
1 pearl is 1 animal product. So create 100 pearls need magnitudes: +2group and +1 size.
After... yes, magi are pretty unconcerned with mundane wealth... or not, as all is about "HOW" do you spend your wealth.
What??? How much is a pearl worth? Oh, maybe not all that much.
According to City and Guild it would be 3 pounds of silver for an ounce of superior pearls. Pearls weigh based on size and the size listings I found were not that helpful but the smaller the cheaper. How much size would said magically created pearls come in? If you are talking pearls of a cm across, which isn't that big 0.03 ounce so would be worth around 5 shillings. So it really depends what size and weight pearls the spell makes, and the ranges I've seen listed go from 2mm to 20mm as for normally occuring pearls. If every pearl created was the maximum average in size at 20mm then each would be worth 1/4 pound of silver or so.
That's definately one way to make them less open to abuse, any innkeep would never consider paying back change for that amount as how would he know the value, it makes them so valuable that only wealthy merchants would accept them accept at a large discount.
(Never suggested "asking for change" - how plebian. The magi are giving them away as if they are ignorant of or jaded to their value - which in one sense is true. They show the pearl(s), say "will this cover our debt?", and walk away.)
ONLY if they did so NON-locally.
Unless trying to sell huge quantities, unlikely. The Arabian pearl markets alone could easily absorb pearls worth a few hundred pounds per year.
Split the main sales between the North African, Italian and Arabian markets and disperse the rest to smaller merchants and you should be able to sell for a few hundred pounds per year with zero effect on the market in general.
Hardly, as already said, pearls are not money. IF you sell too much in a single local area, their value will drop there, but thats nothing uncommon or odd.
And IF you want stories about it, there´s plenty enough ways to make that happen anyway.
Actually that’s not how the individual and group targets work.
If you cast an individual targeted spell you can destroy, change, create one individual up to the base size determined by the realm. (In animals case it’s the size of a pony.) If it’s bigger then the base size you have to increase the magnitude of the spell but there is generally no bonus for the target being smaller. So If you cast a create pearl spell that created an individual pearl you would get just one pearl (albeit it could be a pretty big one).
Now group spells affect a group of targets that mass no more then ten times a base individual. They however can be split up any way you desire. For herbam for instance you could target two medium sized trees, ten bushes, or a whole field of grass.
So my spell created a group, ten times the mass of a pony, of pearls, a very small item. I conservatively set the actual mass at one ton because C&G already did the math out for what that was worth. Still that’s a lot of pearls.
A saga I had started playing in some time ago (it quickly folded due to other commitments of part of the troupe) put an interesting twist on wealth and the Order - I suspect influenced to some extent by the Continuum rpg cosmology.
Since it's quite easy to mass produce wealth with magic, but you really need a trade network to make that wealth satisfy the needs of the average covenant without creating problems with the local economy ... the natural solution is to ask the Redcaps. That's one more service that makes the Redcaps so useful - they'll get you whatever you need. And since once you have the trade network it takes just very little magic to produce enough wealth to sustain the entire Order for decades, eventually the Redcaps are not just acquiring goods for covenants for a price - they where acquiring them forfree*, for any magus in good standing in the Order. Do you need fine blown glassware for your lab? A gem for your talisman? Damascus steel for your grogs? Grain? Wool? Pottery? Ask the Redcaps and you'll get them within a season, compliments of House Mercere, no need to worry about the trifling details of mundanity.
Of course, that one time when House Mercere asks you that little favour - a vote at a Tribunal, access to your library etc. - how could you think of refusing? And there are rumors - oh, false rumors, spread by mages who have been marched for diabolism and interference with mundanes and endangering the Order - that there are subtle enchantments weaved into those goods, or perhaps simply arcane connections to them kept in deep, secret cellars at Harco. Let's face it: Tremere was a dilettante.
Actually, that's exactly how it works for animal products. Look at the Base 5 CrAn guidelines - an "Individual" is one thing for animal products - "...an Individual is a single hair, a single hide, or a single tusk." Or a single pearl.
The general Animal Guidelines say "A basic Individual for Animal is an animal of about the same size of a pony..." - but the guidelines to create an animal product (like a pearl) are diff.
And pearls are pretty much Animal more than Terram. (CrAn is MUCH tougher than CrTe, as a rule).
But the concept could work as well with any semi-precious gem, which is where pearls are usually categorized and would be of about the same value and "spending" considerations.
That would be true if we we talking about an Individual target spell, but the spell in question has a Group target. That's not how the Group target works.
Group makes a group of individuals equal to the mass of Ten base individuals for the Form. The whole mass is divided amongst any number of individuals. The base individual for Animal form is a pony sized thing. Ten ponies is a lot of pearls.
So does the spell “Weaver’s trap of webs” create only ten strands of spider silk with a mass of less then ten normal strands. Or does it create a large network of webs with a mass smaller then 10 ponies. Because in the description of the spell in the book on page 117 it clearly compares the size of the group that spell creates to ten “basic individuals”
I think this spell is clear evidence that while the Level 5 effect it uses “create an animal product” does create a single tusk, hair, or hide if it just targets an individual. When however, the effect has a group target the total mass it produces is still compared to ten times the standard base individual (the aforementioned pony).
THis is how we have been using the guidelines as well. They might not be 100% consistent, though, but I find that the "mistake" might be with the "only one animal product created" for the base individual, not the group version 
Serf's parma, though
Maine (Xavi posted between i read and i post) No. Group is mass x10 of the individual, but the individual is "the single animal product" (ONE hair, ONE pearl). So with base 5 +2 group, you create 10 pearls.
The base animal is general, but specific individual (as "individual animal product") are specifics, so they are coming first. It's a traditionnal law rule: "lex specialis derogat generali".
The funny thing is that the animal product has no consistent size. It can be a tusk, a hair, an eye, a leg, a head, a 5 feet unicorn horn or an inch horse molar. Great variations in size between individuals. If it is a dragon claw it could be as large as a pony, or it can be as small as a herring spine. I find that applying the pony size for group targets is more consistent since the "individual product" guideline does not seem very consistent itself. It is a catch-all category, and as such it has some problems if you look too closely at it.
I find that a hair is Individual and a pelt is also Individual to be slightly disturbing. Still, I understand it since a catch-all approach was needed with something as diverse as animals, but I still think that basic the group size on 10x pony size is more consistent.
CHeers,
Xavi
A Group Target CrAn spell to make pearls can make up to ten ponies' mass of pearls. The size of the base Individual applies to all uses of a Form, so an elephant needs a size modifier, and so might a dragon's hide. Similarly, Group is multiple items with a mass no larger than ten times a base Individual. The note on the products means that an Individual is one product, rather than what you would get off a single animal of the appropriate type. It's important that it says "Individual", not "base Individual". An elephant is an Individual, just a big one.
So what's the point? If with a "base individual" you could go to (say) 100kgs, what's the point of a "individual product" being 1 hair? With the base 5 you can create one product.
So with a group, you can create 10x mass of one product...
Either the individual product has no sense either it has.
If it has, it's like i said IMC.
If it hasn't, with 1 spell, you could with base 5 create the hair of 100kg, so the hair for +- 100 persons (and i'm not generous there), so, much more hair than a single spell could produce in the first conception. And that would mean that with the base product, you create more product (100kg wool eg) than one individual (1 sheep say 50kg of flesh and 50kf of wool) can wear...
I stay with my previous statement which really make sense.
An "Individual" is not a measure of mass, it's one, concrete, individual thing. Creo Rabbit with Target Individual creates one rabbit. Creo Grasshopper with Target Individual creates one grasshopper. Creo Wool with Target Individual creates one strand of wool. You could, perhaps, create a really long strand of wool, but not one so long it couldn't exist naturally, so not one strand at 100kg.
An Individual is one thing, no matter how big or small it is. The base Individual sets the maximum mass it can have without needing additional magnitudes. A Group is lots of things, and the size limit is determined by the total mass. If the individuals are very small, you can have a lot of them.