Many of us are fans of Ting, which is an ideal MVNO for for people who don't use a lot of data. (Real mustachians use wifi everywhere they can.) I rarely use the carrier services so my phone bill is under $14 every month. (The minimum bill varies by state due to local taxes, fess, and whatnot.)
You'll need to have or buy a particular Sprint-compatible phone (but you don't have to get it from Ting, you can get them used) but then the service is dirt-cheap. I have a Samsung Galaxy Nexus which is under $100 on the used market these days but if you want the latest and greatest, the new Google Nexus phone is not a bad deal and can be used on Ting.
There are many Ting threads here if you're looking for more information. The Ting website is pretty informative and their customer service simply rocks. There's a Ting referral thread on here somewhere, if you use one of the links then you get an immediate $25 credit on your account before you've even bought anything. (I can PM you my referral link if you wish, but I don't want to flog it here because I think there are rules against that.)
Good luck!