Cost is one thing. A drop is another.
I don't think there's a problem with the Pats' receivers clogging the middle, though -- and if there were, it wouldn't be simply a Welker problem. (That sounds a bit like the notion of baserunners clogging the basepaths in baseball.) If that were a problem, the biggest issue would be with Lloyd, imo, because he doesn't bring to the table what many of us hoped he would.
But given the team's output on offense this year, and Welker's role in that offense, I don't see how there's any way to conclude he is a problem.
So bringing this back, yes, of course his cost will prove critical. My point was that singling Welker out for a drop tonight in light of everything else he has done all year, including tonight, is silly.