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![](https://lemmy.world/pictrs/image/10bbd270-bdb3-44f4-86fb-08ae957e571a.png)
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).
The extra material/weight needed for the same/better strength in PVC becomes an advantage. The fittings do seem expensive until you compare the cost to 3D print equivalents. Any joints you don’t want to wobble, you cement.
She was joking about putting a cot in it, after shelving and plants, and yeah, it seemed roomy enough to me.