It's difficult to evaluate these CBs in a vacuum because the Patriots have outstanding safeties. McCourty and Harmon are easily the best FS combo in the league (most teams don't even have two FS on the field at once.). Harmon woukd start on many teams while McCourty is without question a perennial all-pro caliber player.
I've brought up a few times that Butler got absolutely roasted in SB51. He gave up numerous big plays, had an inexcusably horrible PI call in 3rd down (just before Atlanta went up 28-3), and was badly beaten by a no name for a possible 70-yard TD had Hightower not stip sacked Ryan. Rowe had tremendous coverage on Jones in SB51 despite Jones being 4/4. Ryan and Rowe both came on very strong towards the end of last season.
Butler's struggles seem to have continued this year. I don't know if it's injuries, age (he is 27, which is not young for a CB), schematic changes, attitude, etc. I don't think he's a bad CB by any means, but the line between him and the rest of the Patriots CBs has been blurry for some time now. I use Chris Harris Jr. as a benchmark player for a smaller CB like Butler, and I don't think Butler is even close to Harris.
Anyway, I've always appreciated Butler and wish he could remain a Patriot, but I think it's clear the Patriots aren't even in the same ball park from a contract perspective. They already made their decision when they signed Gilmore to a monster deal. There is no way they are spending $13M on Gilmore, $9M on McCourty, $5M on Harmon, and then forking over another huge deal to another CB when they have other inexpensive assets (Rowe, Jones) who are developing nicely.
