Fighters purse is subject to the following deductions (in britain)

Trainer - 10% of gross purse ( this applies to most contests but seperate arrangements of a fixed fee are now made when purses reach a high level).

Manager - 25% of remaining purse after all reasonable expenses that the boxer has incurred (trainer , cutsman, etc) have been deducted.

This is the BBBC standard formula. I don't know how they do it elsewhere.