I don't have a pallet coop, but am very much into re-purposing things. My coop only cost me time. The extra wood and siding, and other items were things I had laying around. It is not big, but sufficient for my pets.
Given the choice of using pallets I would use old wooden fencing for recycle builds. Whole lot less work and nails when recycling. Used both and fence slats are longer and less prone to split.