Doubt it's much about money, but rather him feeling he's still good enough to still play on some level and not be used as a reserve player here, as he would. Going to MLS is not purely about money anymore, there's increasingly more people interested in football there, it's a nice country to migrate to (and he can be quite unnoticed in public, at least compared to in Europe) and now there's the opportunity to be part of the start up, of a new team there with a big potential. All around good move for him it seems like. He's a legend and will be remembered as one of Englands best midfielders through the times, but his age is showing and it's probably for the best if he leaves.