I lucked out on mine.
My first coop I built for about 25 bucks. It's treated The Oldies well but will be replace in a few weeks with a coop I recently built. I choose simple designs to avoid the headaches, though.
The second coop has had several donations made towards it from contractor buddies, like sheets of wood and a 5 gallon bucket of paint. The total cost has been about 150 bucks. That includes a 24 sq ft coop and a 64 sq ft run, the wire to wrap it, hardware, wood, a couple new drill bits, etc.
I've found that it's not so much the cost, but the reward of building your own coop. My young daughters (9 & 8) have helped in the process and have learned some valuable skills.