I believe Ting (Sprint MVNO) will also support the Nexus 5, with LTE support as well. You must get a SIM from them, however.
For those who do not know, it is a pay-for-what-you-use kind of thing with varying dollar amounts depending on usage (Minutes, SMS, Data). Not the cheapest if you are a heavy data user, as 3GB is $60, and each MB past that is 2.25 cents. But if you want to use the phone and don't use much data, it can be cheap.