I disagree. I think it is a total waste of time to train anyone with less than allstar potential and if you can you could buy guys with superstar potential. The market has changed a lot in the last few seasons and you can get dirt cheap trainees now.
It took me less than a minute to find some great trainees that you could buy for less than $50,000. Set age max 19, potential min allstat. Height no more than 6'6". Salary $4,000 minimum. Skills of OD and PA (the core skills) minimum average. Maximum current bid $100,000. Currently 16 guys like that for sale, some suck but several are amazing but I am not allowed to link to players on sale here. But you can work it out.