Get some 2X4′s and build a frame. You can buy thick rolls of plastic at any hardware store. Cover you frame with this plastic. Staple or nail the plastic to the frame. Leave one side open..so that you can get inside. You can frame a door and cover it in plastic or make a plastic roll up door. Just make you will be able to completely close it at night so that your plants do not freeze. Hope this helps.

Look up “greenhouse plans + low budget” on your search engine.

happygurl had a good answer, but if you want to save even more, buy rough cut lumber, it is cheaper.

happygurl has the right idea,but to make it even cheaper use 2×2′s instead of 2×4′s. another way to save is frame it out then go around and find people who are throwing out old windows and use them over your frame.

I would think the 2&4′s and sheet plastic would run you about $60.00 for a small greenhouse. Upgrade to low-grade clear corrugated plastic and you’ll get some more durability. You might still be able to keep it under $120.
Make sure it’s secured down so it wont blow away.

Use 1/2 inch PVC plumbing tubing and angle fittings. The tube will flex enough if want to make hoops and it is strong enough if you want to make it angular. The fittings come in 45 , 90 degree angles, and t- fittings. You don’t have to saw and nail lumber, the tube cuts easily with a hacksaw. A PVC frame can be glued or dissasembled and modified too. Cover it with roll plastic. You can easily make a greenhouse in an afternoon for under $75.