I can tell you it was starting off real cheap for me ($8), but I've ended up spending about $400 so far, which is including the 6X20 run I haven't built yet. I'll bet I spend another $200-300 to finish the coop and run which isn't bad considering how much lumber I've salvaged.
All of my lumber I had salvaged for free, and the trailer I put my coop on was free as well. So far I've paid for roofing, hinges, latches, hardware cloth, vinyl flooring, and other misc. The run I plan on making out of electrical PVC pipe, which is UV resistant and will be half circle shaped. Then covering with hardware cloth.
It seems like the little things add up quick.
My coop is 6X10, and is being built on an old sand rail trailer. It will accomodate 24 chickens by the time I'm done. I want the ability to move the coop around on our 5 acres. Funny thing is, we don't even live there yet. The 1939 farm house that is there is currently being remodeled and will hopefully be done by this late fall or early spring. My wife got too excited and decided to go get a dozen chicks which are currently 7 weeks old now living in a 2X4 box in our garage.
I've been taking pictures and will start my own BYC page when it's finally finished.
It's taken me 3 weekends so far, but I've enjoyed doing it (frustrating at times though ;-) ). I'd say if you're fairly handy and have the time, build it. Although I think buying a built one wouldn't be that much more. Building materials have gone up in price quite a bit, so stuff adds up quick.
Good Luck!
Zak
PS I've never built anything like this before, but I do have general construction knowledge background, which has helped. And my plans were basically from some pictures of other coops with the same design.