A lot really depends on climate and other local considerations like predators. In warm climate with no land predators in the area a "decent" coop can be a leanto shelter made out of scrap materials surrounded by some net fencing (or no fencing). Overbuilding can be a liability because of reduced ventilation and more habitat for mold and parasites. In very cold climates or areas with lots of four legged predators, however, the minimum requirements may be totally different. One other reason prefab coops are often a poor choice--the ideal coop depends so much on local environment, management differences, and other individual factors...
(Sorry, I realize thats not a directly helpful answer to your question, but these things do make a big difference in defining what a good coop is or what's a decent value for you.)