This is my greenhouse experiment. The 'high-tunnel' greenhouse design is just about the cheapest to build because it requires very little in the way of materials. This will be a 10' x 24' greenhouse built for around $500 over a few weekends.
The tricky part in this design lies in finding precisely the right size of 3/4" PVC pipe. In this case, it's the inside diameter that we're concerned about. Measure your pipe carefully (before you buy it) and get the stuff that is between 29/32" and 15/16" ID. A lot of it will be 7/8 but skip those because you will not be able to insert the metal conduit. You want the stuff with the widest opening so the metal conduit (aided with a bit of WD-40) will slide in from 6" to 24" giving a lot of wiggle room in the positioning while giving a snug fit. I didn't find any that were too big.
Flip through the gallery below to see part 1 (of 4?) of the construction process - framing the structure.