What’s the cheapest unlocked smartphone that is compatible with both CDMA and GSM providers?

Basically the title: What’s the cheapest unlocked smartphone that is compatible with both CDMA and GSM providers?

I’m looking for a cheap, new smartphone, that would be compatible with all of the U.S. cell phone providers (CDMA and GSM), for backup purposes.

Any suggestions would be welcome! Thanks in advance.

tinyurlis.gdv.gdv.htu.nuclck.ruulvis.netshrtco.detny.im