At first, my reaction was like "REALLY???"
After looking into it, I believe this is the right move.
According to spotrac, $13.3 puts him about WR #5, between Dez/Demaryius and T.Y. That's likely what he'd get on the open market if he blew up next year. If he doesn't do well, he'd likely get an Alshon-type 1 year deal for $9 million. Only downside is if the Bills just can't get a deal done, but they have the franchise tag for that scenario.