yikes.... I like small but that is a lot of cash
Certainly small options exist without going over $400, this is what I use:
Intel D201GLY2A Little Valley with 1.33 GHz Celeron Processor (Fan Cooled)
store price = $75 plus shipping for a board that is 6.75 inches (171.45 mm) square
Don't get me wrong, small low power is cool and all. Just saying for a bunch less money I was able to completely hide my system. Sure that is what I wanted.
If you are after a really small system (like this one) then enjoy. The lower power simplifies the PS choices as well. If money was not an issue I would be more interested.... it is all about trade offs for me.