Right now the least expensive phone you can buy right now that FirefoxOS definitely works on (that I am aware of) is the ZTE Open.
There isn't any official effort from Mozilla to port to existing devices that I know of, but there is a porting guide if you want to get your hands dirty: https://developer.mozilla.org/en-US/Firefox_OS/Porting
I know that existing Android ROM hacking communities have made some progress too, but I personally use a phone that explicitly supports FirefoxOS (Geeksphone Keon) which I am pretty happy with. I'll probably switch to the new reference device (the Flame) once it is available, although I'm really interested in the $25 Spreadtrum model too.
Also according to Mozilla there are several Nexus phones that can run FFOS [2]
[1] http://shop.geeksphone.com/es/moviles/9-revolution.html
[2] https://developer.mozilla.org/en-US/Firefox_OS/Phones#Firefo...