Say you have a cheesemaking grog with a magical buffalo that will never go dry, essentially a level 25 power. How much money does it bring the covenant? Would it be 5 pounds because it's Magnitude 5, or would it be 25 pounds because the limitless & permanent creation of milk is more akin to ritual Creo magic?
Does the milk supply produce a certain amount of milk in a period, or does it produce an infinite amount of milk?
How much cheese can the cheesemaker make if the cheesemaker can make all the cheese he can?
By strict interpretation of the Covenants rule (p. 67, Magic), I suspect it saves £5 per year as a magic item, if you use it that way. This is, I stress, cost savings, not income.
Remaining, exportable cheese can be sold (and it may be more profitable to sell as much cheese as possible, rather than save the £5.) Foodstuff in general, for bulk sale, sells for about £10 to the ton, £20 for superior, and £50 for expensive (p.141, City & Guild). Going for the expensive end of the market may be best, as you will quickly meet demand. In any case, figure out how much you make, and do the math from there.
If you are using the Covenants rules, your costs for the cheesemaker and supplies are covered in those numbers; if you sell directly, your profits are very high to begin with. If you sell to distributors, your profits are lower but your sales will probably be higher. Selling to distributors also spreads the cheese around.
If you make and sell a lot of cheese, you can expect to depress the cheese market, undercutting your profits, and promoting poverty for commercial cheese producers, which in time may lead to undesirable results, like a mob of cheesed peasants lobbing runny yard cheese pots at you, the Evil Wizards Who Make the Devil's Rennet. 
Of possibly greater concern is the widely accepted but not universally Coded rule on wizards magically enriching themselves. Magically created cheese is still magically created, cheesemaker or no.
Well, after the results of Stonehenge and causing the silver collapse, it's opening up one's covenant to interfering with mundanes and thereby bringing ruin upon one's sodales. Magi should be aware that mas producing any item and attempting to sell it causes problems in the local economy, and doing so brings trouble. So much better to make problems in a remote economy and make it a Grand Tribunal issue... 
Does the rule against interference/enrichment apply to the woman or her buffalo? Neither are products of Hermetic effort, unless you count giving them a place to stay and sell their product.
Sophistry.
Is the woman selling her cheese directly? Is she then paying rents to the magi? And if so, why? If the covenant's benefiting from the income, of said buffalo and cheesmaker, they are doing much more than just providing a place to stay and sell the product.
Presumably, a cow that never goes dry is a big deal and would likely be stolen from a simple cheesmaker, so the covenant is offering her protection, too.
Sophistry on my part, or yours? Your logic indicates that income from a pawnbroker, a swordsmith's shop, or anyone else for that matter is breaking the rules; because they're offered the same things this cheesemaker would be.
The sophistry in your argument is that you initially stated it was the magi doing this, because one of their covenfolk had the buffalo. In the middle ages, the lord probably owns the buffalo, not the grog. Maybe the magi are the lords in this case, because of %reasons%. Changing it and saying that the covenant doesn't own the buffalo, that the woman does, while the magi derive all of the income benefits from said buffalo is sophistry. A Tribunal will call sophistry if any covenant who uses the excuse that it wasn't their buffalo if it can be shown that said buffalo was the source of their income.
But, yeah, magi can get into all kinds of trouble, if their actions (magical or otherwise) destabilize the economy. The prohibition is interfering with "mundanes and thereby bring ruin upon my sodales." And Magi can do that with or without magic, magic will just have the tendency to make it happen faster, or make it more likely.
I should clarify, that I don't see selling 5 or 25 mythic pounds worth of magical cheese as destabilizing. Just that the argument that it's the covenant isn't deriving a benefit from these operations is sophistry. Could a 25 mythic pound operation be considered destabilizing, might it be cause for defending at Tribunal? No, not IMO. But if that operation can be scaled up, say mating the buffalo to another cow, and having 8 or 10 of the cows that now never go dry, using Hermetic magic to make the buffalo want to be milked all the time and also improve overall production, that's the kind of thing that will get investigated, IMO.
I had not changed a thing in my argument. I had never claimed the buffalo was created by the magi, and any responsibility of it and the grog's behavior is as much as any other covenfolk. You're the one making the argument that having a person/family provide an income for the covenant is against the rules; because a wealthy Jewish family is certainly financially more significant than an ever-flowing farm animal.
The unlimited milk would not speed up the cheese making process. They are still making a fine Stilton ( for example) they just need a lot more space. The local economy can only handle so much fine Stilton cheese so making 20 large wheels when the locals only can handle 2 would be a problem. Now the Covenant would have to move the cheese to make any profit. They must fine buyers who would love to have a large wheel of Stilton and buy another in say 6 weeks when another batch is ready.
So the local economy would not be hurt much unless they were crushing the other cheese makers and setting up a cheese monopoly but even then, i find a large influx of cheese to be more of a problem for the Covenant than bringing ruin upon my sodales.
Eh? It will bring the covenant 0 income, then. First, because the grog owns the buffalo. And the grog is then generating the income. Your argument's premise is that the covenant is making income from the cow, owned by the grog...
In point of fact your OP was:
Sophistry (see, I can be dismissive too). My question was whether the bison brought 5 pounds or 25, so trying to claim that a grog/beast (independently magical from any magi action) is different from a skilled swordsmith both in savings/income or Hermetic Code is already moving away from my question.
Ok, I'm not being dismissive. Your OP was a question as to how much income does it bring. Tim (correctly) pointed out that it doesn't bring income, and is instead, cost savings. He then speculated that if it were income and your covenant was manufacturing a large amount of cheese that it might fall within the "widely accepted but not universally Coded rule on magi enriching themselves."
My initial response in the thread was to him, and it was about mass producing any item (and I didn't specify that it need be made magically) and causing problems for the local economy was a problem that could be prosecuted under the Code. You then asked the question about whether it applied to the woman/buffalo and whether if it wasn't as a result of a Hermetic effect if it mattered. But that wasn't in your OP, (the grog claiming the income) nor was it germane to the point I made. It was the magi/covenant involved in the selling, deriving the income, as you indicated in the OP. If they, the magi/covenant receive the income, then it's sophistry to say that the magi/covenant isn't involved if they get all of the income but aren't responsible for the problems they cause. "Oh no, that's not my cow, Presiding Quaesitor, our grog owns it, it's just responsible for making us all this money." That argument is an example of sophistry. And it doesn't matter if it is a Hermetic effect or not, not really. Involving magic has a tendency to accentuate the problem by making it an acute presentation of the problem. It's perfectly legitimate, under the Code, to claim interfering by other magi encroaching into your covenant's primary economic activity and affecting the market. Notice I said claim. It can probably be defended against, but that is beside the point.
Now, if this were a question of cost savings, I would go with 5 pounds, because man cannot live on cheese alone. There, I answered your question directly, and apologize for responding to TimOB's comment before your OP. Nevertheless, your subsequent posts make this a murky subject, at best, because if this is an income generation scheme, I'd say it has logistic challenges, of how much time one cow can be milked (tolerate the activity), and how much cheese it can produce. A gallon of milk produces a pound of cheese. Water buffalo, as a rule, don't produce a lot of milk, let's say that it never grows dry, but only 8 gallons of milk can be acquired per day, so that equates to about 8 lbs of cheese/day. There are, when you take into account saints days and not working sundays, about 200 working days per year, so your operation might produce 1,600 pounds of cheese per year. Or something. That sounds about right for 5 pounds of covenant savings.
Is the cheesemaker covenfolk? Is she simply a local woman who resides nearby and provides the magi with a great surplus of cheese in exchange for her upkeep? (Dodgy, but it might dodge.)
If she is a resident, and her expenses are paid by the covenant, she's effectively covenfolk, therefore if trouble arises the magi are hooked for it.
However, I could see where if the cow and cheesemaker were established before or completely aside from the efforts of the covenant, and the covenant is only the distributor of the cheese (they're the only ones willing to deal with the hunchbacked and evil-eyed madwoman, or whatever), this seems quite defensible. The covenant buys the cheese at a sinfully low price per weight, keeps some, and sells the rest - the quaesitors may look askance, but that seems tolerable, if exploitative. The magi are not first parties (they did not enchant the cow or hasten the cheese ripening), and not second parties (they did not provide enchanted cheesemaking tools).
If she is paying a rent, the magi are landlords. So long as the local rents, and taxes, and tithes, and bribes, are paid, and the nobles and church and the notoriously vicious Cheesemaker's Guild isn't challenged, this might slide by.
It should also be pointed out that generally the code allows making money through magic, it is when you destabilize an economy by producing too many precious metals that they start to get worried. I seriously doubt anyone is going to bring ruin upon their soldales by running down the price of cheese.
It should also be noted that, unless living in a chartered town, a landlord is a lord, meaning that he is and can be held somehow responsible for his subject's actions, as they are not his clients but rather more like his vassals/property. Just a side note.