Kolb hasn't had a particularly successful career, but it's been lucrative. You'll remember that he signed a 1-year, $12 million extension with the Eagles in 2010. Then when he was traded to the Cardinals where he signed a 5-year $65 million deal with $12 million guarnateed.
Point is, Kolb has earned a lot of money in his career and will likely continue to pull in decent bucks as he continues on in the backup circuit for another few years. He'll be 29 at the start of next season, so it's certainly possible that he'll play another 3-5 years. And if he grabs on with the Bills next year, he probably has a decent shot to start for a while. As it stands now, they've got Tavaris Jackson under contract and will almost certainly draft someone... but you could see Kolb getting a chance to play for a while while the rookie gets ready.
Clearly the Cardinals seemed to have gotten the worse end of the trade that swapped Kolb and Dominique Rodgers-Cromartie. Neither team got particularly good play out of the respective players, but at least the Eagles didn't have to shell out megabucks in the process. Plus, they've still got Vinny Curry & Brandon Boykin, both products of the 2nd rounder received in the trade, who could turn into players.