The $19 smartphone: LG K3 2017 from US Cellular
Normally, the LG K3 2017 costs $90, but US Cellular takes $70 off its price via an online discount, so you can purchase the handset for just $19.99. The K3 2017 is available on pre-pay, which means there's no contract involved, though you will need a Simple Connect plan (starting at $35 per month) to actually use the phone.
As you may have guessed, the LG K3 2017 is not a technological marvel, but it should be able to satisfy your basic smartphone needs. The handset sports a 4.5-inch display with 480 x 854 pixels, is powered by a quad-core Snapdragon 210 processor, and runs Android 6.0.1 Marshmallow. There's LTE connectivity on board, plus Wi-Fi, GPS, 1 GB of RAM, and 8 GB of storage space (expandable via a microSD card slot). The new phone also features a 5 MP rear camera with flash, a 2 MP front-facing camera, and a 2100 mAh battery.