Yubikey NEO NFC
works with the
LG Volt (LS740) which is one of the few budget pre-paid phones (the only one I know of that works with Sprint and ringplus+) that have NFC capability at the time of this post (August 2015). Can be found for as little as $30 when on sale.
That said, It can take quite a few tries before the NEO is recognized. It takes some moving around, swiping and/or holding to find the sweet spot and have the key recognized. It can work through a pair of jeans pocket but it is even trickier to get it to recognize the NEO without knowing exactly where it is in your pocket. Which could make you look quite foolish if you are fiddling around with your phone in one hand and trying to find your NEO inside your pocket with the other.
Given that some of the expensive "flagship" phones exhibit the same problems I'm not going to complain about the LG Volt that I paid a mere $35 for.
As with any new device I assume I'm hoping I will get more proficient at getting it to be recognized the more I use it.
Great phone to use with your NEO especially if you are on a budget.