From Rotoworld, via ESPN: "Mack, 28, will likely be the Browns' top priority and could be franchise-tagged, but CEO Joe Banner may not believe in doling out that kind of cash to a center. Mack is going to be a hot commodity on the open market and is easily the top center available in free agency -- he graded out as Pro Football Focus' No. 1 center in 2013. He'll probably aim for a deal similar to the one Ryan Kalil signed with the Panthers at six years and $49.1 million with $19 million guaranteed. Mack has previously said he'll give the Browns a chance to match any offer, though that was before the coaching staff was fired after one season."
Let's preface this by saying it only holds water if the Giants are finished with Davis Baas as their starting center. And even if they are, their choices are limited. Mack won't last a week on the street. Half the league will be gunning for this guy. If the Giants want Mack, they'd have to strike quick.