The cost will really depend on the size and how many recycled materials you have or can find. I'd suggest start looking on freecycle and craigslist and try to find as many free and cheap items you can use in the building of your coop. Depending on what you find that will kinda dictate the design. Then make a list of what you still need and go to your local lumber store and get the rest.
I bought and built one of the 89.00 playhouses. its wonderful. I wll try to attach photo.
it is 4 feet wide 4 feet deep and 6 feet high in the center. walls, roof door made of wood, cedar wood. the door is even a dutch door. the wood shutters close.
the windows are a vynal frame , we will wrap plastic around these for the winter.
we will put our bantys in here, maybe broody henwhen she is sitting on a nest.
or other smaller chicks till they get used to the bigger girls or vs versa. they will share a wire fence so they can see each other. we will use smaller wire so the big girls cannot get to the little chicks. I am not sure how many full size chickens this would hold.
square foot per hen ratio..... I still think its a good deal for the money. I have another one comming from another Lowes to my local Lowes. let me know If I can be of any help.