I would sign him for 8-10 mil a year, but nothing more. I agree the c's culture would do wonders for him and any young up and coming player, but not willing to go 12+ mil a year for him.
There will be quite a few big name big men who will be free agents in the next few years
D12
Hibbert
Josh Smith
Ibaka
Big Al
Illyasova
Millsap
Kaman
I would take any of those guys except Kaman and Millsap over McGee
It all depends on the money really .