Amazing. BSD on Sub $20 boards. They list a bunch of hardware that it runs on and also include support for a simulator so you can try it out without hardware.
Considering the requirements, I'd say you could build a BSD machine for less than U$10 (the microcontroller is about US$5). Nowhere near as functional as a Pi, but much smaller.
I'm not sure that's the amazing bit seeing as you can get a Raspberry Pi for not much more than $20.