I don't have a portable pen aka Chicken Tractor, but I did build a coop a few years ago and don't have $100.00 in it total. I used scrap cedar slabs, used tin on the roof, cedar fence pickets for the door. Only thing I bought was some 4x4 lumber, gate hardware and wire mesh.
We keep about 15 layers these days, when we had more dogs, we kept as a many as 60-70 layers and the dogs had fresh eggs daily with their food.
Here's a pic of the coop