The NanoPi R4S is now orderable from FriendlyELEC. The single-board computer starts at US$45 and comes with a Rockchip RK3399 chipset. The device is available with 1 GB or 4 GB of RAM and an optional CNC metal case.
https://www.notebookcheck.net/FriendlyELEC-releases-the-NanoPi-R4S-for-US-45.507562.0.html