my husband built a 4x4 temp coop for our 6 chickens who were 8 weeks at the time (now its my feed storage thingie) for $100. metal roof, t1-11 siding, cheap pylwood floor up on 4 blocks it did its job. also look on craiglist or post an ad on craigslist thats how i got mine for $100. 4feet wide 7and half feet long and 6or7feet tall. got it to hold me over till my dream coop could be built =) good luck
Pallets or go behind stores, esp grocery stores...I found the frame for my 4x4 coop behind a store...LOL go dumpster diving or cruise around after people put their garbage out..I found all my 2x4s that someone was tossing...You'd be surprised what you can find...good luck and show us what you built when it's done!
Check out implement dealers. I have found some large plywood shipping containers behind farm equipment dealers. Some would be big enough for a small coop. Some are the right size to make into nesting boxes or several nesting boxes in one crate. You can get some crates that may contain complete sheets of plywood. Plus there might be some pallets there as well.
We always check out our local construction sites. The crews always have a "discard" pile which they are completely willing to give away! I also went dumpster diving in their bins and found beautiful trimming, a few five dollar hinges, and a lot of plywood/2x4s. Good luck!
The cheapest coop that I can think of is one that friend and I built when we were kids...we stacked straw bails and threw an army surplus tarp over the top of it. The roosts were cut from two inch tree limbs and then shoved into the bails about two feet off the ground.
It wasn't real pretty but it worked just fine. After a few months, there was quite a population of grubs and bugs in the straw...very happy chickens.
Like what everyone else has mentioned, you can use pallets. If you want to go the extra mile, you can take the pallets apart and overlap the thin panels and use it to look like fancy siding for your coop. Just an idea.