http://overthecap.com/salary-cap-space
Cards are projected to be $9.8M over the 2015 salary cap heading into the 2015 season. When Carson Palmer's contract voids following the Super Bowl, they'll get $10M in cap space back, which would put them right at the cap.
Which means that Larry Fitzgerald will again be holding the team hostage going into another offseason. This'll be the second consecutive offseason where the Cards will be forced to deal with Fitzgerald's contract, and the fifth or sixth in Fitz's career. The Cards probably hold more leverage in the deal since Fitzgerald probably won't be able to get the $16M guaranteed that Fitzgerald is scheduled to make from anyone else, but the Cards have given up leverage before without getting much in return.
It'd be really, really difficult to imagine Carson Palmer coming back to the Cards in 2015 without substantially reducing Fitzgerald's salary while keeping him on the roster. Cutting Fitzgerald outright would create $14M+ in dead money while giving the Cards cap relief of $9.2M.
A successful season by Palmer would probably mean a deal in the range of three years, $45M with $20M guaranteed.