It depends on a lot of factors. How good do you envision your trainee eventually becoming? Are you planning on training him every week? Will you be training him on a 1 position basis a lot (e.g. PG passing or pressure)?
Without knowing the answers to those questions, I can only generalize. If you are training a guard whom you expect to be a mainstay in your starting lineup for years to come, I would say that you should go with star potential or higher. That would give you a max salary of over 30k.
For a big man, you'll need All-Star potential at a minimum. For SF, you might be able to squeak by with a starter (potential), but star would be better.