Pre-Owned or Refurbished iPhone 7 $339
You can buy a refurbished 7 from Walmart or a used iPhone 7 from Gazelle and pay around $340 for an iPhone 7.
The iPhone 7 is a recent iPhone and it includes many of the latest features like Apple Pay, 3D Touch, an improved camera, support for the Apple Watch, a faster processor and it supports iOS 12 plus a few more updates. This iPhone is more expensive, but it will allow you to use the phone longer and get more updates than the other cheap iPhones on this list.
Walmart sells a refurbished iPhone 7 for as low as $249. There are various models, but usually, there is at least stock of the iPhone 7 for one major U.S. carrier.
You can also buy a refurbished iPhone 7 direct from Apple with a one year warranty included. These start at $469 direct from Apple.
Neither of these options includes a full Apple warranty, or the ability to buy AppleCare+, so factor that into the cost compared to the best iPhone 7 deals.