If you guys can figure out how to build a dead silent box to run Boxee in anywhere near the same footprint for even close to $200, let me know. I don't think it can be done by a long shot.
The BeagleBoard could do it but the XBMC port isn't quite there yet last time I checked.
At the end of the day, you'll be spending a lot of money and time replicating something you can buy for $200 even if it is does come in an odd shape enclosure so you have to decide whether it is worth it for you. You'll get a lot more options and freedom with your own box but you'll pay for that with time and money.
I assume that other manufacturers will come out with Boxee boxes anyway and you won't be restricted to the D-Link cube.