I can currently buy a Zotac Atom 510 ("Ion2") box for $209 before rebates ($15 mail-in), a 1GB stick of DDR2 for $19, and a very large 2.5" drive for it for about $50.

So.. the modern price point for these roll-your-own XBMC / MythTV boxes is now about $300, before remote control. A good USB HDTV tuner for recording costs another $50 or so, but that's not an option at all for the closed boxes.

Getting there..