The question is how much more Fayne adds than Volchenkov and Salvador, both of whom you'd be paying more, and if you'd be comfortable giving Fayne more minutes than he's gotten, especially when you take into account that he would presumably make that up by getting PK time. (It's also possible that Fayne's average icetime looks low because he was in DeBoer's doghouse for the first month or two of the season).
I don't know the nuts and bolts, but it really seems like the Devils PK is close to being a set of interchangeable parts. I mean, I guess Volchenkov adds a shot blocking element, but I don't see what Salvador brings that can't be taught to a more skilled player.
As much as I/we rip Salvador and Volchenkov, they are very good on the PK. There are certain instincts out there that can only be refined with experience. By the way, I don't think Fayne coming back is contingent on a buyout. He is a RH and I suspect both he and Larsson can't both exist on the roster. If they think Larsson can replace what he does, than I doubt he is back