US superstore chain WalMart are selling off the Lumia 435 with a prepay T-Mobile sim for just $10 and the Lumia 640 with a prepay AT&T sim for just $29.99. At the time of writing this the 435 is out of stock, not surprising at that price, but customers can register to be informed when it’s available again. The 640 is available.
Both the Lumia 435 and 640 run Windows Phone 8.1 out of the box and should be upgraded to Windows 10 Mobile at some point. The 640 has a 5 inch 720p display with an 8MP rear camera and 0.9MP front camera, while the 435 has a 4 inch 480p display, a 2MP rear camera and VGA front camera.