Of course there are ways! Determining reasonable compensation for services is something that is done in business all of the time and even the tax law has established guidelines for that issue.
This sort of thing appears to have been done already for the smaller stuff (personal appearances, autographs, etc.). I looked at the Providence basketball NIL site and it was very similar to the Gophers' site in that prices were assigned based on the estimated stature of the player.
For the bigger stuff, that can be done too. Supposedly Iowa's Caitlin Clark has multiple very lucrative NIL deals with big companies. That is understandable because she is likely the most well-known and celebrated women's basketball star. But, for lesser-knowns, one could use prevailing rates paid to obscure actors, models, spokespersons, etc.
Establishing reasonable amounts for services will take some effort but, if you're going to have rules, you have to have guidelines for interpretation and penalties for violations.