He's not going anywhere for cheap and could still be in Detroit 3 more years. He'd get two 1st for him but I don't think he's the guy you wanna trade. But I guess if Stafford did ruin his shoulder then getting two 1st for Calvin wouldn't be the worst thing since they would need a new QB anyway.
Detroit is so close to turning the corner though, next year with the right off-season they can make the wildcard. All of this depends on the health and progression of Stafford.