Radxa's new version of the Rock Pi 4, the Model C, is finally orderable. The Rock Pi 4 Model C has an M.2 connector for NVMe storage and is available for as little as US$59. The SBC is available with a hexa-core processor and 4 GB of RAM.
https://www.notebookcheck.net/Rock-Pi-4-Model-C-A-US-59-single-board-computer-that-supports-NVMe-drives-and-PoE.483801.0.html