Ok so think that buying a higher trainer is useless unless he can add value to your player's value.
So how about this-u buy an advanced trainer having 13k salary for 50k and u use him to train your players for 3 seasons. So u would be paying somewhere about 450k more than the basic trainer.but then by that time your player would have had at least 25 pops.
Let's say he was a big man with all skills at average so at the end of 3 seasons he would have a skill set of all sensationals.i think that a 21 y.o wih such skills can easily fetch u above 500k plus he would win u matches in lower divisions with his skills.so I don't see any loss in such kinda deal.and if u train a guard then he will fetch u much more price at the market.
I am not saying that monoskilled players should be trained but I am just giving u another scenario.