This one came from Costco, it was about $400. It's 6 x 6 x 6, and it's plex. Very flimsy in reality, but once it's all together it's pretty good. The flex panels slide into the aluminum bars.
It's this brand, but not this model. (Costco apparently had them special made, a little shorter and slightly different than their regular line)
We levelled the ground, then put down a weed barrier and pea gravel, as you said. We then put down the aluminum base which comes with the greenhouse, and squared it. We built the greenhouse in a different area of the yard because you need a lot of elbow room. It took four or five of us to carry it over and attach it to the base, which was then staked down to the ground. Finally, it got silicone caulking to seal the roof. It took us most of a day to do.
It works really well, and I'm very pleased. If you are in an area with a lot of wind, I would consider also some stronger attachments for it, as it is pretty light in relationship to its surface area. A really serious wind might do some damage. You could easily make some tie-downs if required. And you really do need the shadecloth. It gets darned warm in there when the weather is nice. I leave the door and vent open all night now, in the spring, and it's just right because it keeps the humidity slightly higher and the temp slightly warmer.
Every morning, I come in and sprinkle the gravel (which, despite the weed barrier, grows thin stalks of grass that get pulled) then "mist" the plants. Then each gets water according to its need. They need less water than the outside plants, so they don't actually get watered as frequently. I'm going to set it up with a mist system soon, a cheap and simple one, so that come summer the mister can be turned on easily. And the cuttings are doing better in the greenhouse than they did outside, but a bottom heating mat is also on the shopping list. You can drill through the plex to run wires and hoses if needed.
Let us know what you decide to buy!
Joanie