Mexes has his flaws and inconsistencies, but for 12m and when the options are likes of Bonnucci and unknown gambles or possibly mediocre ones, it would be obvious upgrade to get him (would prefer Kjaer of those mentionned, but the others not so much).
Btw, I really would have liked to have signed Rannocchia, decent talent, but HUGE he isnt, heard be mentionned a while back, bit weird to read honestly.