When I was a kid we had a popup. Everything above the 2' high sidewall was canvas. Had to snap a canvas cover over the whole thing before it went down the road (no hard top). No sink, no potty, no fridge, nothing but an empty floor. Basically a tent mounted on a trailer. Much of the increase in price is probably all the "extras" found in popups like fridges, stoves, slide outs, A/C unit, etc.... It's probably easier to put all that in a fixed wall trailer than make it so it open up, rises up, etc.. for you to put the canvas panels in.
Just like a basic pickup truck. Can't find one because the manufactures have figured out their profit margin is highest on all the add ons and fancy trim packages. They did the same thing to the humble popup.
