Value is a good guide, but not always. The managers above you sell the free youth they get, and their prices are usually fairly high, but they are *not* fast trainers, they're ducks. Basically the fastest trainers are from fictional sounding team names, with very even attributes, and extremely high price tag.

Also remember to plan ahead, if you are training an extra role or SA, do it BEFORE you add Q to the player, they all slow down the better their Q.