I made a spreadsheet for my girlfriend when she bought the iPhone 5s. She was debating between AT&T, StraightTalk, and AIO wireless. On AIO, you're still better off on AT&T over the course of a 2 year contract by $90. If you choose StraightTalk, you'll save $174 over 2 years. (That savings goes up to $299 if you buy an AT&T phone and then immediately cancel and pay the ETF; both prices combined are less than the $749 sticker price.)

The trick is that most people don't replace their phones on a schedule. Some phones don't hold enough of a charge after 18 months and get replaced early; some people are still happy with their 4 year old phone. If you are on a pay-as-you-go plan and hold onto a phone for longer than 2 years, the savings start to really add up.

The other trick is that many people can get pay-as-you-go prices on contract, through their corporation, credit union, or university. When I was on AT&T in 2009, I used my university's alumni association to bring my cost down to $55 per month.