You could unlock the iPhone and go to T-Mobile. Since you wouldn't need a new phone, you could get one of their Even More Plus plans which are $10 cheaper per month ($20 if you have a data plan) and include no phone subsidy. The only thing you'd lose out on would be 3G, but without a data plan, being on 2G likely wouldn't be that much of an issue, and you could still use the Internet stuff on Wi-Fi.