There isn't a fee - he is out of contract.
Strange one, Carlos. He was reallly popular with the fans (hence the shenanigans at his last game for us), but over five years, no Villa manager played him consistently at centre half.
MON stuck him at RB for massive periods of time. He's decent defensively, but terrible coming forward. He's also the only Spanish person on the planet with no ball control. His first touch is like watching a puppy try to pick up a balloon.
Top bloke, though, always gives 100 percent.
Always looked a good CB to me, tbf.