I've seen single board computers on Ebay for as little as $20
Search for "sbc" under 'computers and networking' catagory.
But if the board dosen't have everything (like sound) you'd need a backplane to connect the spc to isa/pci cards, which would make your computer as large as a regular desktop PC (so make sure the sbc you're buying has 'everything' on board)
...or maybe someone knows the pinouts to connect an additional card to an sbc with solder and wire?
Also be aware that you may have to solder a female molex connector to power it as many sbcs have funky power connectors.
Dig it?
Craig
(pictured below is an sbc in a chassis, seen from above, with a backplane that has isa,pci and atx headers)