It's possible to train your guy in a way to keep your trainee cheap, my u21 guards have been 32-34k at the start of their respective campaigns, around 50k the next.
As long as you train him in a balanced way, you will have around 6/7 seasons to move up before you have to sell, just make sure you expand your arena capacity and invest in a level 5 coach.
It depends on how you want to train your player, an level 7 potential can be maxed out around 23/24.