Training speed is the biggest factor from what I understand.
If you see an 18-19yr old that is inexpensive, it's because he will just train at normal speeds. Where as if you see one that looks like a really crazy price, he'll train super fast.

A good way to use this is to get a couple of the super fast trainers early in the season and build them up! Get a good one and he'll last many seasons at a high level.