Yeah I get your point about price estimation. However, about talent I have to disagree. O.Torres is not compatible with the Simeone's style. His Porto loan stint was very good, so he has showed some sign at top level. 21 is very young for central midfielder, especially those who games based on play making.
I don't believe it's entirely a matter of difference in style. Koke doesn't work super hard on defense, Griezz has a free-ish role up front. Cholo will let you do your thing on offence if you're good enough.
IMO his Porto stint was so-so - in a way, similar in a way to Mata's time at United. He's shown he's capable of turning around games, but he doesn't do it regularly. You have a point that he's still young, but in order to develop, he needs to play regularly and he's at an age where he needs to be allowed to shoulder more responsibilities for the team.
Again, not doubting his ability but his stock has clearly taken a tumble since he was Spain's next young, bright hope in CM. He needs to start proving himself, be it at Atleti or somewhere else.
Why would they sell Torres? Especially basically for peanuts
It's basically potential for potential. Oliver Torres' stock in 2016 is not as high as it was in 2013. Mangala could pan out for Atleti, while Torres could pan out for City. Will be a decent risk to take for both parties.