I don't think we have anything besides what is in the guidelines:
Magi charge a very high price for teaching another magus for a season (the equivalent of 4 to 10 pawns of vis).
I'd set the price according to the maximum TeFo of the magus and how rare are the spells he is teaching, not to the spell level. An archmage won't stop his personal projects for a meager amount of vis, and will charge more if it's a customized spell that you can't find elsewhere. But I think you could get a small discount from your hermetic patron as a favor, or even trade for service if it seems reasonable?
@Nithyn you kinda of lost me again. The blend between Volkhv and hermetic magic would result in hermetic theurgy, is that it? What is exactly the relation with the faerie? I understand that hermetic theurgy deals with the magic realm, at least as written.
I'll get a read on the section about Spirits and Daimons from Mysteries and RoP:M. Is that the kind of thing you'd be interested in interacting with?