My trainer charged %15. I think 10% is the average. I never went pro, but 15% is what my trainer charged his pros.
Trainers shouldn't be getting more than 5%, it's just as bad as promoters and managers getting 20-30%, these fighters are getting robbed blind and the sad thing is that many don't care and just let it happen. No fighter should be getting 50% of the purse, they are actually the ones getting punched in the face. It's a disgrace.
Trainers standard fee is 10%, managers is 30% boxer stays with 60% wihch is pretty messed up boxers should get more than 60 !
A good trainer and manager is worth every penny. There is more to boxing than the 4-12 rounds of the fight.
It's like everything else in boxing. Negotiable. How much time the trainer has available? How many pros does he train? How much is the boxer willing to pay?