Keltham listens to Yaisa's presentation of how much she doesn't actually know any way to arrive at the fair value with the fascinated look of somebody who's suddenly, actually appreciating that huge amounts of childhood training he went through were actually there for an important reason and accomplished anything.
Keltham says that he will, if you ask him about any price that isn't a large fraction of his overall wealth and reliable expected future income, give the same answer to 'how much would you pay to get this extra' and 'how much additional payment would you forgo to get this extra' and 'how much would you pay to avoid losing this' and 'how much additional payment would you demand if you were losing this'. Because, like, if you didn't, your trade patterns would go in exploitable circles, depending on how much money you had and how people asked you various questions.
It leads to really large differences that matter in everyday financial life, so everybody, not just Keepers, trains until the asymmetries between financial gains and financial losses mostly go away. If you have time to think, you can just consider all four questions and average the result, for example, to guarantee that you'll give the same answer no matter the form of the question.
There's all sorts of mental exercises for figuring out where to start pricing things, if you don't know where to start! Find something that seems about equally valuable to you, equally exciting or happiness-making to you, which already has a legible price, and ask, not how much you pay for that, but how much you would pay for that.
Some amount of words later, he'll get around to asking Yaisa her Fair price for Keltham doing what he did, and her Fair price to be his for a week. Well, once he's quickly, before the spell wears off, explained to her the notion of the least price that makes her feel positively cheerful about the transaction, and not just that the transaction was fair - hopefully this spell enables that part too, but if the spell-price doesn't sound cheerful to her, she should say so, obviously.