Be sure to check the amount of skill points on the player; there are players with 2,5k salary that have 50+ SP, and can therefore be great U21 candidates. I bought a Superstar with 51 SP for just 15k a few seasons ago. I don't know if the prices for USA trainees are higher because of the demand or lower because of the supply, but the point I'm making is there are bargains if you look closely :)