Woot is currently blowing out a slew of Galaxy S5's that are unlocked. These are priced at $99 and will work on GSM networks as they are GSM unlocked and don't include CDMA radios unfortunately. This is a great price for a smartphone that is still pretty great, even though it is nearly 3 years old now, it'll still get the job done.
The Galaxy S5 was announced back in 2014, so it is a bit old and does feature somewhat old specs, but it's still more than capable – that's the great thing about flagship devices. The Galaxy S5 sports a 5.1-inch 1920×1080 resolution display, powered by the Qualcomm Snapdragon 801 processor with 2GB of RAM and 16GB of storage. So it is a bit on the light side for performance, but Android is built to run on far less RAM, so you'll still be good to go. There's a removable battery here, which has a capacity of 2800mAh, and there is also a micro SD card slot so you can expand storage – which is now up to 256GB, at release it was only 64GB.
Woot is selling these today for $99, until they are sold out. At this price, it'll likely sell out fairly fast, so you'll want to make sure you grab one before its gone. Woot does offer up free shipping, however, despite being an Amazon company, it does not do free two-day shipping for Prime members, so you'll be waiting a bit longer, unfortunately.