Covenants gives rules for the trade of books. There has been a lot of discussion on these boards about how the economics of that would work and how common trades would be of tractatus.
I have not seen any rules for the trade in lab texts. The core rulebook seems to consider copies of lab texts as being a common thing for covenant libraries to contain, and the rules pretty much allow any magus to produce lab texts fro trade. I have not seen anything giving rules of thumb of how easy or difficult it is to get the lab text for any particular spell.
I was considering having my magi learn Whispering Winds. The spell description says that spell fits poorly into Hermetic Theory. Without a lab text, does it even make sense for my magus to invent this spell?
Obviously, it will depend on the Storyguide, and the input of those who run stories, but I was curious about what people thought about this.
Good question, I don't have a rule; but here are some thoughts. My suss is to make the rarity of the lab text linked to the magnitude and degree of power needed for completion of the described activity. Use a scale that ramps up harshly at the rare end. I'd like to think that these types of trades are very common (run of the mill) between parens and close alliances.
A lab text created when a magus invests a spell is probably of far lower value than creating anything new. Also a low power spell that can easily be spont'ed is going to be worth far less, but a Conjuring the Mystic Tower is probably very valuable due to the usefulness, magnitude and rarity.
A counter-point to this is probably that lab texts might be copied easily, so you could allow the "trade" to be a magus visiting to transcribe a rare volume.
I'd say the default unit of currency is vis or an agreed favor. A favor has the special touch of adding a hook into the character that is not easily ignored later. If given the option as a player and GM I'd go for a hook everytime.
I've been kicking around ideas for "fair trade" between covenants of books/lab texts. Using Quality and Level...
ROUGH GUIDELINES
Summa divided into lvls
=1-8, 9-16, 17+ (each category =1p of vis of appropriate type Fo/Te)
=Q1-5, no addition. Q6-9, +1. Q10+, +2 or more. (Natural quality)
=Scribe, Illumination, Binding all provide +1 to the cost each. (figuring these types of things affect the value and time invested in book).
Tractatus
=Just use the Q guidelines above.
=+1p for any two specials (Scribe/Illum etc)
LAB TEXT
1p=5 magnitudes
+1p for spells greater than 4mag/lvl20 (SG discretion on whether spell is rare or not).
(Theory being that spells around 4mag are relatively common, inexpensive.)
So a covenant decides to trade a Summa of Creo (L12, Q10+Scribe). This could be sold for approx. 5p (+2 for lvl12, +2 for Q10, +1 Scribe).
As noted previously... I have an hour commute so when I say Rough Guideline, I mean driving at 70mph, eyes on the road, brain in the clouds "Rough."
Guardians of the Forest has some guidelines for the availability of Lab Texts in the Great Library at Durenmar. The interesting point it that the Great Library contains ALL "standard" spells (those in the core rulebook, one assumes) and a version at every level, up to 40, of all "standard" general spells. Availability there is a question of whether someone else has already checked them out, not whether they exist.
There are also rules for the availability of non-standard spells, but they can best be summarized as "the ST decides on a likelihood and rolls a dice", which isn't that helpful.
Still, based on the first part, it is safe to assume that all standard spells are easily available, provided the magus is willing to wait a few seasons to get them.
As for Whispering Winds, I think you really need a Lab Text for this one (or a teacher), since it is unique. However it is also in the core rulebook, meaning it is well-known in the Order, and present in a large number of libraries.