Mother Earth News August/September 2011 issue, page 97, had a great idea for building a coop out of a metal shed. You could probably find one of the old metal sheds on Craigslist for free or extremely cheap. Don't need to worry about the side panels so much as long as the roof is good.
Here's a link to their Clever Shed-to-Coop Conversion
Here's a link to their Clever Shed-to-Coop Conversion