OP, if you want to get into riding fixed cheaply, I recommend the BikesDirect route, too. Your best bet for a cheap bike is to get one at Goodwill or at a garage sale or Craigslist. But you'll have to spend money to convert it to fixed and that often means the total bill is higher than the price of a new Kilo from BD.

If you do choose to convert an older bike, be sure to read http://sheldonbrown.com/fixed/index.html and Fixed Gear on the Cheap

Any bike being sold on eBay as "PERFECT FOR FIXIE!!!!! TRACK!!!" will be overpriced by the rich hipsters or be some gas pipe junk.