In another forum someone was asking how to make a sensor network more cheaply, with reference to the "Raspberry Shake" and "Raspberry Boom" devices, which use an R-Pi as controller plus add-on board with 24-bit 100 Hz ADC. The computing power needed for this use is low, so any of the versions of R-Pi work, including the original one. I was suggesting you can just buy early R-Pi boards cheap used, because most people want the latest and assume the old models are obsolete. Then I checked eBay and found used RPi boards are not so common, and the cheapest "buy it now" price I saw was around US$18 (!).
I have several older RPi boards I'm not using, and surely I'm not alone. Is there some other market I don't know about, or does the RPi really hold its value better than any other kind of computer?