In order to be worth training, a new trainee needs to have pretty good starting skills. A salary of $3500+ might be an ok guideline, but that's very rough, and each player is a different case. In general, one thing to consider heavily is skill level in main areas. It's really good if the key skills for a trainee are already at or near respectable to start. If the player has any major holes (say, atrocious passing for a guard), then you need to think about whether the hole(s) can be fixed (sometimes the answer is yes, sometimes no). It's also really good if the player has secondaries that you like, since these are hard to work on. If you can start out with a PF trainee with respectable JS and OD, for example, then that can be a huge advantage.
http://www.buzzerbeater.com/community/forum/read.aspx?thread=144856&m=32