I highly doubt anyone is going to be making a trade for one of the worst coverage safeties in the game today. And I am not certain but I would predict the cap hit would be pretty big if the Cowboys cut him, that is why I believe Williams will be playing a role similar to Archuleta's, the last year he was with the skins, a safety playing only situational downs, where they can't hurt the team.
I didn't even think about the cap possibilities of them cutting him. And obviously no one is oging to trade for him so yeah it might be cost-effective and just plain smart just to keep him on the roster for another year.