A few years ago a friend got several of the big watermelon bins from stores for free. These are about 5'x5' and 3 feet tall wood boxes. They just needed a little bleach, a good spray down, and a few days in the sun. I helped him build 4 coops out of 8 bins in just a couple days.
Pallets. Lots of people use pallets. Anything you can scavenge that people don't want. Anything you can get off Craigslist cheap, like sheds, playhouses, dog houses, dog kennels, windows or any other materials. Stuff from Freecycle.
We just finished our coop. Most of it was made out of pallets and cull wood from Home Depot. We did purchase new 2x4's for the framing so it'd be solid and a few fencing pickets to fill in where the pallet wood ran out. Total we spent under $50. The biggest expense we invested in was a clear polycarbonate (I think) piece for part of the roof. We wanted the chicks to have sunlight even before we let them out.
We made our coop out of an old trampoline that had a hole in the tarp. My hubby bought pipe the same size to make it taller then went around it with chicken wire. We can move it around the yard with enough people to help and the roosts are easy to install because it is round. We did add another waterproof tarp to the top and top half of the pen is covered with scrap opaque plastic to provide privacy for the roosts. Our nesting boxes are various plastic tubs and boxes that have lost lids or holes in them to make them otherwise unusable and milk crates. Different hens like different boxes.
So, our coop was made from mostly recycled items. We live in South Texas and have very mild winters so we don't have to worry too much about winterizing, if we are expecting a cold spell we will turn on the heat lamps and put tarps up all around the coop to protect them from the wind. We have to worry more about the heat in the summer here and having a coop that allows adequate air flow is important and keeps my birds happy.