Greenhouse kits are also expensive. The creator of this Instructables has created a 7 ft. high x 10 ft. deep x 6 ft. wide greenhouse from windows salvaged from his neighbors, who were preforming upgrades. The only new materials were lumber, frame, screws, caulk and latches.
Though it seemed like a bit of a puzzle to assemble the windows, the end product is truly unique and 100% functional.
See the complete Instructables here.