While I don't love PFR's
Approximate Value metric, it's a good baseline for a discussion like this, which combines minor contributions to a team (which being a second or third QB would provide) with long periods of time. McCown's AV is 40. His career comps are Mark Wilson, Mike Tomczak, Mike Livingston, and Mike Phipps. I've heard of one of those guys.
Matt Cassel's AV is 47.
Jay Cutler's is 86.
Nick Foles's is 32 (hilariously, his AV in the year he lead the Eagles to the Super Bowl is 2, but the year he went 4-7 as a starter for the Rams (a thing that apparently happened) was 3).
Derek Carr's is 82.