It's hardly an exact science cause comparing one position to another is hard, however, if you forced me to put a number on it...I'd say that if you take a center and a winger that are very comparable as to what they can do for you offensively, are there aren't any egregious deficiencies (skating, complete lack of any defensive awareness) then I think a center is perhaps 10% more valuable financially. The extra responsibilities defensively and the faceoff responsibilities are the first two things to come to mind.
In Mitch's case, yes, he did penalty kill...but the Leafs PK was pretty awful last season. Was that his fault? I can't say...as I don't have the data in front of me...maybe it wasn't at all...but he is going to get lumped into that bad PK just by association.
I still think he should fall into the $9-9.5 AAV on a 3 year. If he wants $10m+ then we're talking 6+ years.