Best Sprint MVNO is Ting. You, essentially, pay for what you use. My bill averages about $20 a month. There are some limitations on which phones you can use. Most Android flagship phones are available at launch. For things like the iPhone, Sprint has to give permission for Ting to activate them. The general rule is that once it has been a year since the phone came out on the Sprint network, you can take the used device to Sprint. Here is their latest update about upcoming devices:
https://ting.com/blog/device-update-didja-miss-us-2/
They even have a signup for notifications when the iPhone 5S/C is available.