The ultimate cheap greenhouse, which I built two or three of on different homesteads in Georgia, cost me nothing but a few staples, a bit of tape, and a couple of hours' worth of burning a candle.
The frame was green saplings, wire-tied to metal stakes pounded into the ground, and bent to overlap one another as arches overhead....8 or 10 feet tall, and wired together, along with several horizontal poles.
The cover was two layers of plastic, made of mattress and furniture bags from the mattress and furniture store dumpsters. These goods are shipped in huge plastic bags, which can be cut open into large square or rectangular pieces. These can be "welded" together by folding the edges of two pieces together and passing them through a candle flame, thus making any size plastic desired. Small holes can be patched with clear plastic packing tape. Two such pieces were put up over the frame and weighed down with logs on either side, and some excess at one end gathered together and tucked into the frame and weighed down in cold, or opened up on a warm day, with the other end stapled to the door of the cabin, or in one case the opening of a large wall tent. Some scrounged baler twine going from log to log over the top of the plastic helped keep it tight and less likely to be thrashed in a wind. By pulling the plastic down in warm weather and folding it up out of the sun, I could get two or sometimes three winters' use out of it. Not only did I start many seedlings and winter potted plants on benches inside of this, but I could grow larger plants right in the ground.