Here's what I said about McCann elsewhere.
He'll need to put up something like 9.4 WAR or ~1.8 a year over the life of the contract. Steamer has him at 3.6 wins next year which would be paying 5 million/win which is pretty nice, covers a good deal of the contract (38% or so). If you project him at like a 0.5 win decline, he'd provide $32 million in surplus value. Although I don't know if that's too conservative, cause who knows if he can stick at catcher and that gives him like a win's worth of value, and if he moves to DH that would be going from +12.5 runs/600 to -17.5 runs/600. That's a few wins. Putting it in perspective, if he had DH'd this year and had a full season, he would have been worth approximately 1 win.
TL;DR could be a nice contract but needs to stick at catcher for it to be mostly worth it which seems unlikely.