At this point in time, if we tried to sell Martinez we would probably only come away with enough to buy Pepe - roughly in the range of 7.5M, or so.
It's generally not a good idea to purchase a player and then sell him at a loss after one season - he's had a ton of fitness issues, and the team has generally been a complete mess. There are a lot of issues in play here, it's complicated... but I think it depends a great deal on our next coach.
If we end up playing a 4-3-3 next year, Martinez could provide depth that we would otherwise have to secure on the market. Considering we've already spent the cash, I wouldn't mind seeing this guy given another chance.
It's an absolute joke that he was bought as a 4-4-2 winger.