David LaFerney has made a great post on how to build a green house for $50 here http://doorgarden.com/10/50-dollar-hoop-house-green-house
He also has a post on how to build a great door for it here http://doorgarden.com/11/building-greenhouse-doors and he also shares he experiances with his new greenhouse 6 months later here http://doorgarden.com/04/6-months-in-the-greenhouse
Enjoy!
