The least expensive bike I own:
Mercier Kilo WT - $429 new (now BD is charging $499 for the same bike)
Replacement wheels after I trashed the originals: $65 (Nashbar cheap-o 32 spoke track wheels)
SKS Chromoplastic Fenders: $50
Replacement no-name stem as the stock one was too long: $20
Shimano 540 SPD pedals: $50
Total: $614
BTW, if you want an inexpensive, no frills steel single speed that can fit 35mm tires with ease, look no further. I've put thousands of miles on this bike without issue.