Just check the staff market, a lvl 4 trainer has salary of around 13k, a lvl 5 trainer has around 25k (it can be lower but the buy price will be 200k+), and a lvl 6 trainer is 30k I believe (haven't checked for what prices they go, maybe it's more).
So say you can buy a 28k lvl 5 trainer for ~100k as opposed to a 13k lvl 4 trainer for 25k, do you think a having your pops come 3-5 weeks earlier after 4 seasons of training is worth the 900k? (14 weeks x 4 seasons x 15k difference in salary + 75k buying price).