I just used what I managed to pick up from the side of the road on a hard rubbish collection day, heaps of scrap wood lengths and some ply. Old pallets can be really useful depending on the size of coop/how many birds you want.
I re-purposed a playhouse, re-used 'junk' wood, saved windows, 2x4s, and bought oops paint from home depot, and my 4x7 coop still cost quite a bit. If (o.k, when I build another coop, I would try to builtd a pallet coop using whole pallets screwed together for the walls, and save any windows and scrap wood I com across so I only have to buy extra 2x4s, shingles, OSB, and hardware. If you look around in the coop pages or do a search, a lot of minimum cost coops can be found. Many people swear by re-purposing old sheds, dog houses, play houses, office furniture.... almost anything wooden can be turned into a coop if you are creative enough and/or want bantams.