They Will always lose value, as i said before fast trainers lose about 25% of there value, but they also gaine them back in less that 2 weeks.

If a player value is 2M he Will decrease to 1,5M, but If you had raised Q instead of teach him SA His value would be higher maybe 2,5M so His actual value would be around 1,88 M, that means you would have a higher value player.
Of course that a player with SA whend put in the auction have higher chances of a bidding war.