Yes. Trust is the issue.
I have seen people approach the pricing structure all sorts of ways -- from the pro quoting a lump sum with no justification to the pro and am working out the fees, line by line, with the am's budget running the show, so to speak. The real issue is as you say, ee, one of whether the am trusts the pro. It's up to the pro to act in a way that her/his students find trustworthy.