There's all kinds of options. I built my first one out of a truck box cap (free with broken windows) a lot of fence lumber ( free) and chain link fencing (0.20 /lb scrap yard) basically built a frame the same sise as the truck cap and mounted it on top then used chain link fence as a dig proofing skirt.
My next one is a old gazebo frame ( free since no canvas on it ). Should be a simple build. Just got to scrounge up some wall and fence material. Kijiji, Craig's list, facebook groups, are all a good source of materials. Scrap yards are my favourites since you can get metal fencing and siding by weight instead of paying by piece.
I build based on materials I can get. There might be weeks before I can get something I need or I may see something I'll need later and have to get it now. I built my first run out of pallets till I got enough fencing and posts then built the new one around the old one. Now that's its painted and complete it looks as good as anything I can build with new materials. Chickens are not picky give them what they need and constantly improve things for them.