The store-bought(/online-purchased) ones actually tend to have metal frames. Plastic(PVC, usually) strong enough to support even a flimsy structure tends to be more expensive(although more flexible and weather-proof) than metal tubing, particularly when you consider increased weight and volume requirements, increasing both packaging and shipping costs.

My wife recently ordered a 6.5ft x 6.5ft x 10ft one for about $80, shipped. Biggest concern we have so far is the need to strap it down so it doesn't blow away. It came with double-layered sheeting with a built-in airgap(think doubled-up bubble-wrap, only it feels like vinyl).

I live in a colder climate (frost-free growing season is June 1 - September 1), so I use store bought and homemade small greenhouses on my outdoor gardens to extend my growing season. Here are my thoughts:

• Moisture: They are decent at keeping things more humid. However if the garden is outside and in direct sun then on warm days you’d need to open up the sides some to let heat out, which would also let the moisture out. Otherwise the plants get cooked as the greenhouses get very hot. I have a pitcher plant indoors that I keep in a tiny greenhouse to keep the humidity up and that works well but it’s not in direct sun
• Shade: I’ve seen commercial ginseng farms that use shade cloth over the plants because ginseng usually grows in the woods and so doesn’t like much direct sunlight. Shade cloth may be a better option for the sunrot. Or a shade cloth over a greenhouse - I haven’t tried that so I don’t know how much the shade cloth would change how hot the greenhouse gets