My fixie cost very little.
I got an old road bike off 'freecycle' with a screw on block. Bought a fixed sprocket and new chain. It needed a few bits from the spares box (seatpost and saddle) and I was good to go.
I enjoyed it but didn't get on with the very narrow road bars on it so I treated it to some nice Nitto bullbars and some £6 aero brake levers from my LBS's bargain bin.
I enjoyed it even more, particularly for my commute to the extent that I've decided to throw some money at a new one, possibly a Kona Paddy Wagon or Kinesis Convert2. I reckon my commute costs me £5+ per day by car so (very roughly) if I commute by bike once a week for 2 years, it'll pay for itself. I do realise I'm kidding myself here. I already commute about once a week on average on my geared road bike so to pay for the fixie I'd really have to be commuting twice a week (unlikely) or saving £500 of Ultegra cassettes and mechs over two years (also unlikely)