How much does the iPhone 5 cost monthly (AT&T)?

I'm going to buy it with the 2 year plan

AT&Ts website explains the pricing. It totally depends on your plan. Shared data plans (like mine) cost less per phone the higher your data. Mine is $70 for the 4 G of data plus $40 for each phone (we have two iPhones).

Depends on the voice plan that you use and how much data you want the cheapest data is 20 bucks a month and it gives you 300 MB