The unlocked iPhone 5 has now finally made its way into the US store. It is available in prices that have been expected (and also leaked). The iPhone is available in for the following prices:
-$649 for a 16 GB iPhone
-$749 for a 32 GB iPhone
-$849 for a 64 GB iPhone
The prices are lower than in any other Apple Online Store in any other country. Canada is the closest to the US' price but this is the cheapest.
The unlocked iPhone 5 will only work on CDMA based carriers and not GSMA so in the US it will not work on Verizon or Sprint. Below is the full note from Apple's website: