Virgin Mobile began offering the iPhone 5s and iPhone 5c yesterday, and this is quite likely the least expensive option for having an iPhone. Virgin is a no-contract carrier, meaning that you pay for the total cost of the phone up front and then pay as little as $35 per month for service. This lowest tier includes unlimited text and data, and 300 anytime minutes of calling. To take the bite out of that initial cost, Virgin Mobile is selling the unsubsidized phones for $100 less than Apple sells them. You can get the 16GB iPhone 5s for $550 and the 16GB iPhone 5c for $450.
The tradeoff, of course, is that they use the same network as Sprint, which has less coverage than the larger carriers such as Verizon and AT&T. You can see a map of their coverage here. If you tend to mostly use your phone in a limited area, this is an excellent choice. Be aware, though, that your phone won't be compatible with Verizon, AT&T, or T-Mobile, so you won't be able to switch carriers later on. A number of the new iPhone models are already out of stock on their site, and some of the configurations are only available online. The 16GB models are available both in stores and online.