I have trained a lot. With a lvl 6 trainer, buying the trainer, paying his wage and replace him on the right time.
I keep track of how much I spend a season on training.
Over more than 20 seasons, my average cost is around 800k per season for a trainer.
3 trainees a season, means around 250k to 300k for a player. Not always full training possible, so the ones that receive full training always should be allocated a higher part of the trainers cost.
So the cost for getting your trainnee( draft also cost!) plus the number of seasons times the allocated trainer cost gives you an indication for how much a player should cost.
I hope this helps you.
Only in the dark, we succeed!