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).