Vlad Rose :
Was just looking at the one you shown. And that doesn't include a case, PSU, or storage either.
Actually, on hardkernel.com, they say of the XU3:
Note!! The package includes a XU3 board, a plastic case, an active cooler, micro HDMI cable (as of 2nd of February, 2015) and 5V/4A PSU.
So, we just need to add storage. Let's use 64 GB of eMMC, for $79. Now, you could go smaller, but this will let us spec a comparable PC. Final price: $258.
For the PC: ASRock Q1900M ($62), 2x1 GB DDR3-1333 ($40), mini-ITX case ($38), 64 GB SSD ($48), PSU ($35). Final price: $223. And that's for a pretty low-end system. If I were building it, I know I'd go with a nicer case & PSU, bigger SSD, and more RAM.
My point isn't that the XU3 is cheap. My point is just that it's not easy for PCs to beat that price by a whole lot. Sure, you can do better if you buy used/refurb or scrounge some old parts you might have lying around, but the parts you
do buy would probably be better than the bare minimum.
Anyway, it's kind of a pointless argument. Almost nobody using these devices in a developed country is doing so because they can't scrape together a PC that's at least as powerful. I mean, no matter what, you still need a screen, keyboard, and probably a mouse (if not another computer, entirely, and a network switch + cables). I think the majority of their customers are using them for other reasons.