As much as you can do without without paying more than market average?
The market is very non translucent, unpredictable. But spend money on trainees rather than normal players.
What I thought might be a nice idea but I havnt tried is to buy all your trainees with the same flaw. E.g. Buy perfect Center trainees (tall, high potential, 18 and main skill resp/average) except for one very important skill, which is too low for other managers to consider these guys suitable trainees, hence their price is low. Since all your trainees miss this same skill you can train that very quickly untill they reach resp or something and then suddenly you have great trainees, for little money.