I built a new bike recently for cyclocross racing this fall. It started with an old aluminum Schwinn Fastback CX frame I bought at the Revolution Cycles swap. It's set up as a single speed with a tensioner. It's also set up tubeless. I'm really happy with the way it rides and also how light it is at only 17.8 pounds. It's much faster than anything else I've ever used for cyclocross, despite being somewhat new to single speed. I haven't been riding at all this year but I finished 17/80 in cat 4 on the first day of the Planet Bike Cup in Sun Prairie so I can't complain. Things were looking even better on the second day until I crashed and damaged the bike beyond repair.
My gearing started at 36x16 but I've since moved to 38x16. With 38x16 and the half link it's setup in the magic gear and the tensioner isn't really doing anything as shown below. With 36x16 it needs the tensioner.
The Panaracer Crossblaster tires are not intended for tubeless and neither are the rims. I used three wraps of electrical tape, caffelatex valves, and one cup (the cup included with the bottle) of Stan's fluid per rim. I've been inflating to between 35 and 40 psi. This setup is super light and has lots of traction. The front tire did come off the rim once but it was in an over the bars crash at the USGP where the bars were bent around and one of the brake levers came off! Lesson learned: if your front brake lever comes loose and you don't have brakes, don't keep riding.
Here's what it looked like out at Badger Prairie in Verona:
Here are the details, including prices:
Schwinn Fastback CX frame with seatpost clamp from Revolution swap: $100
Generic Chinese full carbon fork from ebay: $110
FSA RD-80 wheels from a friend: $80
Shimano 105 (?) crankset from a friend: $40
FSA Orbit IS-CX headset from ebay: $37
FSA Compressor steerer tube plug: $14
Panasonic Panaracer CrossBlaster tires from Amazon: $64
Thomson seatpost from ebay: $30
Selle Italia Flite Ti I already had: $0
Cane Creek SCR-5 brake levers I already had: $0
Salsa chainrings (36 and 38 tooth) from a friend: $0
Sette singlespeed conversion kit from PricePoint: $14
SRAM PC 850 8 speed chain (lighter than a single speed chain) from PricePoint: $15
Caffelatex tubeless valves from PricePoint: $15
Alligator cable and housing from PricePoint: $10
Sette bar tape from PricePoint: $8
Ritchey bars from a friend: $5
Soulcraft Convert chain tensioner from a friend: $10
Bontrager Race Lite stem from the swap: $5
Carbon headset spacer from ebay $3
Avid Shorty 4 brakes I already had: $0
Crank Brothers Eggbeaters pedals I already had: $0
So this wasn't exactly a bargain, but it isn't that bad. I probably could have saved myself a lot of hassle by buying something from the swap in February for about $600-1000 but I looked last year and I didn't see anything in that price range that wasn't already well used or really low end spec.