You aren't going to have many options unless you go used.
You can try stealing one. Would be free
What cheapest option do you expect?
You either buy one full price or sign another contract with another line.
If Sprint has good coverage in your area, Virgin Mobile sells them no contract pretty cheaply. Just know that you can't take it away from Virgin Mobile to another carrier.
I don't expect anything?.......I'm just checking to see if there are any similar sales on full priced iPhones. That's all.