We're about to unertake this... fun (how much can we swear at each other anyway?)
We are using part of our 20 foot long lean-to style woodshed as a walk in chicken coop. It will be about 10 feet long, 8 feet wide, and 8 feet tall. The framing and roof are already complete and in place, and we have a TON of scrap lumber, plywood, etc around the property to use to enclose it. There will be two standard size doors hung so we have easy access, as well as to help with cross ventilation in summer months. Total cost? Maybe $50 for random stuff (hinges/screws/etc.) There will be 2 sides with 2 levels each of nestboxes/perches- a total of 18 nestboxes and perches for our 12 chickens. That said, what we are putting together is WAY beyond what they 'need' and into the realm of 'luxury chicken villa' but only because it's what's already available on the property for space/material.
Check around with local builders/contractors/lumberyards to see if you can get some scrap lumber- often it's cheaper to see what you have for material to start with and go from there.
Our 'chicken tractor' will be a large dog pen my dog doesn't like using, with some extra hardware cloth moved around the yard/garden as necessary. Again, this is more space than they need, but it's the material available.