We've gotten a look at what our D looks like without Sherman and without Thomas, and until we've got a reliable option to replace their skills and their leadership, I don't like the idea of the team without them. Bennett, on the other hand, wouldn't fetch us much of anything, I don't think. His age and salary will already have teams expecting the Hawks to cut him rather than trade him, and his outspokenness on issues of race recently could scare away some less progressive owners/management.
As for Lockett, I think he would fetch more than Bennett primarily BECAUSE he's cheap. He's got speed and is a deep ball threat, and if you can get that for a tiny salary cap hit, then I can see teams willing to offer a bit more. I'm not saying I want Lockett to go, but on a team that seems to be determined to get back to a 52/48 run/pass split, I could see them sacrificing a guy like Lockett to add maybe a 3rd rounder that could be used to move toward that run-first, run-often philosophy.