$0 - 1984 Trek 520 I had lying around
$140 - Wheelset with Flipflop Hub
$30 - Sram Chain
$15 - 14t Track cog
$6 - Rimtape
Not bad for a fixed gear, I think. Most other parts (tires, tubes, pedals, crankset, handlebar) I had lying around. Just took it out for my first time on a fixed gear and about tore my legs apart getting used to it. It's definetly different. It's 45x14, and seems to be fine for now, but if it gets to be too much, I might put a 42 on there.
Future upgrades include some real tires, and bullhorn bars.
So thanks to everybody here who helped me out, I've made a few threads asking for advice (and I'm sure I'll have a few more). Pics forthcoming.