You can get At&t Lumia 640 for $30. Go to at&t website and unlock it. You can try windows as back up for fun.
Posted via the Android Central App
As someone who did this last month while I'm waiting for my S7 edge, I can say that you don't need to have att unlock it, just put your SIM card in when before you first turn it on. Att even puts a note in the box telling you to do this.
Pretty terrific phone for $30 too. I got two day battery life out of it consistently. Too bad the App Store is subpar...