Hey, as you can see from my transfer list history, I always take (with only 1 exeption) players for 1k and usualy sell them for about 1-1.5m after three seasons.
I take players who are 18 yo and I look that their JUMP SHOT, JUMP RANGE and OUTSIDE DEFENCE would be at 7 (or at least 6). And then I rotate trains between JS, JR, OD. At the end of the 3rd season those players are like 15-15-15.
U21 teams like to grab them, which increase their value as well.
Not saying that this is the best way, but that is how I am working. :)
P.S., my trainers are both at lvl6