These are the free agent QB's this year:
https://sports.yahoo.com/13-nfl-quarterbacks-free-agents-202717864.html
Spoiler alert, it's Cousins and that's it. He will have market cornered for the teams looking for a free agent QB.
My guess as to what he'll get is 3 years/$125MM (not including any potential BS "void" years and/or fake salary cap shenanigans). He's not a runner, and he's been incredibly durable, so the Achilles injury likely isn't a huge deal if he passes medical exams. Look below at the current QB contracts. Ryan freakin' Tannehill was making $29.5MM/year this year, Derek Carr is at $37.5MM/year and Daniel Jones is at $40MM/year. While those are all just bad signings, it 100% goes into determining Cousins' value as it "sets the market". I would say Matt Stafford seems like a good corollary to Cousins' value.
Length of term is likely going to be the biggest thing he'll be negotiating. A 2 year deal has him re-hitting FA at 38, which is not ideal. He would likely take less money to get a 3 or 4 year deal which would likely be his final FA deal. OR he really values a SB ring and takes less money to sign with a legit contender he sees actually being able to make a run.