Is this 100% supported?
Bonisagus must make their works available - but they don't have to provide the scribing for every mage who shows up begging on their doorstep. So what is scribing worth? A (partial) season of effort for a mage, or for a mundane? One copy to the Order fulfills the requirements, and then it's "Dont bug me, kid!" for all the Order is concerned.
Oooh - I, for my part, am never comfortable with that assumption. Some, certainly - but do you really think that a spell that allows a mage to "talk to animals" is rarer than a spell to talk to stone or air? CrCo that gives a shield grog +5 Strength that much less common than an equal magnitude one that creates a zombie? That no one has circulated a cantrip that lights candles across a room or alters a body but one that lets you analyze ashes or change just the face are standard issue? Eyes of the Cat but not Ears of the Fox? A ward against metal weapons that much less popular than one against wooden ones? The list is endless.
The spells in the book are a shotgun scattering of possible effects, examples and models for how the different Guidelines might be used. There are so many attractive effects in the Guidelines that are just as or more attractive than cannon spells (and no few low-level cannons spells that, imo, can be vastly improved by a simple T/R/D tweak - Sleep with a duration of Diameter, anyone?). Not that they're bad examples - altho' some are fairly obscure - they're clearly just a few approaches out of many, many possible, and some obvious ones are missing exactly because of that - they (or the changes to the ones listed) are obvious, and largely redundant as far as the examples go.
And some Covenants will have very specialized libraries, from members who themselves were specialists and not interested in "general" spells. And, on the other hand, some PC-designed spells, while perfect In Character for that particular mage concept, are far too specialized for general interest, and so not valuable to any who don't share their pursuits.
If you want to run that - hey, you know your saga best. But if you're putting thought into the value of a text, I'd think you'd want to do the same for the intrinsic value of any spell as well.
As has been pointed out, it's hard to stop the copying - it exists, it's going to be used. So the question is of trade, and there are a couple In-Game philosophies that are likely to be encountered.
Many Spring Covenants are desperate, and so copying their texts costs very little - some coin, one pawn of vis, and they're ecstatic. Some larger covenants don't care - and so charge a similarly trivial fee.
The next is "like for like" - you want a spell, give us one we don't have. This is tougher, but a hard-dealing Spring covenant could be like this, or an Autumn that is already large and wants to keep growing, and sees no need for charity.
The price itself is also flexible - some generous and philanthropic covenants might want to see spells shared as much as possible, and only charge the cost of a scribe plus materials. Otoh, highly competitive covenants might realize its' a seller's market, and squeeze every last ducat out of the buyer, charging a mage's wages, and if they don't like it they can go somewhere else! Likewise a Spring Covenant where the magi do their own copying, even if that price is not supportable on the larger market.
In the end, I'd balance all this with 1) whether you want to make any story out of this, or just rubber-stamp it, and b) what makes the best story - both the story of acquiring the texts and the continued story of the covenant (rapid growth or tough survival), and iii) whether you have any players/magi who are excited about horse trading. Don't want to spend an entire session on a text that doesn't excite anyone, but if the group is excited then it can be a sub-plot all its own.