Well for what its worth, here's what I have into my single speed (not fixie, YET). I built it from scratch pretty much. Most of the parts are take-offs from other bikes, including the pedals, cranks, and seatpost, and saddle. Bought new were the front tire and tube and the quill stem. Here's a breakdown of the parts and where I got them:
$20 Centurion Accordo 59cm frame/fork/headset with brakes, LBS
$40 Sugino road double and bottom bracket (used cranks) LBS
$5 26.8mm seatpost, take off from another bike, LBS
$10 Saddle LBS
$25 Suzue Basic hub laced to 27" rim, with tire and tube - BikeForums member
$5 Front wheel rebuild. Front wheel from parts bike
$15 Front tire and tube, Performance Bicycles
$10 BMX pedals, take-offs from LBS
$5 BMX chain
$15 Quill Stem, LBS clearance bin
$0 Flipped and cut drop handlebars from parts bike, brake levers from same bike
$20 Brake hardware and cables, housing, LBS
$20 16T freewheel
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$190 total
Mind you, that's without the fixed gear cog and lockring, which is another $30-40. I figure with all the tools I bought too, I am easily into this $250, when you figure all the little stuff in. And I still have to buy the cog and lockring.
I lucked out on alot of the parts, such as the cranks and seatpost and rear wheel. Those parts alone could have cost triple what they did. But I stumbled onto some great deals. You can too, just be patient. This took me since August to put all together.