The TinyPICO ESP32 development board has been making some waves at the Crowd Supply website. The developer of the board set a very realistic goal of US$100, although the little product has proved to be a very popular project to back. The current total stands at US$19,156 for a development board that measures just 18 mm x 32 mm.
https://www.notebookcheck.net/TinyPICO-The-mighty-ESP32-development-board-that-s-smaller-than-a-thumb.420775.0.html