Well, it's a "confusopoly", but you can reckon to pay about $80 / month for two years, plus an initial $100 - $200, for one single top of the line phone, such as an iPhone 4S.

Over two years, this amortizes the phone cost, and if you keep paying, it's basically just extra gravy for ATT.

In the US most carriers just require you to get one of the plans. The pricing on the plans is independent of which phone you get. So it makes comparing the subsidized prices of the phone an "Apple" to apples thing!