Overpay is when you pay someone more than they are worth, it is a subjective term.
The Bills paid what they had to to secure a marquee player. Nix obviously didn't think he was overpaying.
It is easy to see why some might think that way though, 100 million for 6 years is a staggering amount of money when you think about it. Unless my math is wrong, that's 16.67 million per season. Or put another way, greater than 1 million dollars per regular season game. Just think about that for a minute, that's for one player, and a defensive lineman to boot. Some are going to think that is paying too much, it's their opinion. Not to mention the 50 million guaranteed. It is easy to see why Mr. Williams was all smiles, as was his fiancee I would imagine!