it also depends the time of the season. imho, i've found that when you go shopping trainers in the 12-13 week of the season (first playoff week), you can get lvl 6 trainers of 20-30k salary for about 300k-400k.
don't overestimate either the diff between lvl 4 and 5. look around for Josef Ka's coeffs for trainers work, and also trainee's age effect on decreasing the effectiveness of training. some say it's not worth it the sacrifice (money, plus elapsed time for other duties that would have been completed with that money, like building the arena) for the benefit (a couple of pop per season). if you ask me, i say... it depends. it depends of your plan as manager, most of it. if you have already made your mind and decide to build your team around a developing player, thus tolerating losing some games, and you'll keep this player, make it your franchise player, well, maybe it is worth it. if you're more like a business manager, short term success and stuff, maybe promoting fast -if there's such a thing-, and like buying and selling players while developing your ONE good trainee, or buying a player near his cap and try to cap him with the intention of keeping him, then maybe it's not worth it.
anyway. i say it depends. there's no rule for trainers cost-benefit efficiency. not all costs meet all managing styles, and not all benefits meet them either.