Not everything can be quantified; at least in a manner which we're capable of understanding.
As for Edmunds, he's just sort of OK at everything. Aside from the occasional play where he looks sort of silly, he does an adequate job with most of his responsibilities. I don't think he's near as good an athlete as everybody wants to give him credit for, nor do I think he's a glaring hole in the Bills D.
I just don't want to see us lose a really good player (Poyer, Oliver, Dawkins, etc) because we decided to tie up a bunch of cap space on a guy who can be replaced by a cheaper player who is just.....OK.