Our first tractor was made out of an aluminum frame (nice and light) that he welded together and covered in chicken wire (pretty good against predators). Then for shade/rain protection, he put a roof and 3 wooden sides on half the tractor, thereby making it waaaaay too heavy for me to move by myself. Dave couldn't even drag it around by himself because it caught on the ground too. It was also awkward to work with - being on about 3 feet high inside, I had to practically crawl around in there to fill their food and water. It cost about $25 to build because we already had all of the supplies on hand other than the red paint and hinges. It survived the winter well, so we're keeping it as a back-up for now!
After realizing how heavy the first tractor was, Tractor v2.0 was made out of PVC pipe as a frame (super light), poultry netting to cover it (easily ripped apart by predators), and a tarp for shade/rain protection. The whole thing was really light and easy for me to move around myself. It was still at the awkward 3' height. And it turned out to be *really* flimsy! We had a huge rainstorm, and the tarp gathered about a billion pounds of rain on it, and the whole thing collapsed. Blerg. Because of all the PVC pipe and fittings, plus a big new tarp, this tractor probably cost over $200. *nauseous*
This year, we're building 2 new tractors - hoop houses! The frame is light 1x4s and 2x4s, and 2 cattle panels made of woven wire are bend to make the hoop. Then it's covered in poultry netting. Each one has cost under $100.