Is market value a fixed number for each player or can it increase/decrease depending on whether or not you are training the player?

I'm trying to decide whether or not to waste my efforts power training a high valued young player whose quality is below average on my team. He's a fast trainer (approx +200 in match), but it could take time getting him to reach my team level. Trying to decide whether to just sell him and use that money to buy a higher quality player.