I think you're off on a few points. First off, No one knows how many WRs are going to hit the open market. Wallace is a RFA, so he'll require someone to forfeit a 1st round pick. I highly doubt Welker sees free agency either. NE* will keep him. There's also a legit shot that the chiefs use the franchise tag on Bowe to keep him around. they've mentioned it. Colston and V-Jax seems to be the only "sure" bets to hit free agency along with Stevie. Fewer players will drive prices up.
Also, you're off on your salaries. Look at the per year salary averages for WRs. Fitzgerald makes $14 million per year. Andre Johnon is 12 million. Santonio Holmes is 10 million annually. Brandon Marshall makes 10 million. Calvin Johnson in going into the last year of his rookie deal (11 million per), and they are working on an extension that will probably come in at 13-15 million per season.
Given those numbers, it seems to be an easy bet that Stevie will come in at 8-9 million per year at the low end.