Well, my lovely wife and I just finished up the two roof panels for our little grape arbor/back patio that we are converting into a greenhouse. We've got our first real chance of a frost this week, but a roof and two of those 7-day candles should be enough to keep the plants happy. Even without walls, it is noticeably warmer under the roof, ranging from 2-8 degrees is my estimate, depending on where under the roof. The area is roughly 7' x 14' and has a frame made of landscaping ties, 6 verticals, with an edge running around and a bisecting tie in the center, slightly raised. The panels were constructed of 6, 1" x 2" pressure treated furring strips, covered with one layer of 6-mil polyethylene sheeting from Wally World. The sheeting was attached with a staple-gun, but reinforced with a thin nylon webbing called "greenhouse tape" -- I think it's really duct hanging webbing for that flexible ducting. Rough price calculation, for one panel:
Grand total of $12.41, near as I can figure. Total for all 8 panels would then be $96.88, with a load of scraps left over for other projuects. Oops. forgot the nails and twine. Add $4 to the whole project, max.
The landscaping ties ran $2 each, for a total of $26, and add another $8 for nails and construction adhesive.
So, a medium-small greenhouse for $135 or so. Sooner or later I'll get a cheap thermostat and fan to regulate the temperature better. Right now, I'm just keeping things above freezing.
You may ask: Why panels? Why not just put the sheet directly onto the landscaping ties? Friends, this is Texas, and having a modular greenhouse where the walls can be removed and stored in a relatively small space is a great advantage -- last week we had temperatures of 83F. This week, we get frost.
Plus, this thing doubles as our grape arbor in the summer -- the vines are on the outside so that they can get full chill and what dormancy they can manage ;)