I second what the nino says. Twin wall is great for end walls. It's good as a primary glazing also but doesn't work on curves. Standard twin wall is about a 1.6 r value. Double layer poly is a 1.6 also at a fraction of the price and MUCH MUCH easier to install.
It's not that the twin wall polycarbonate is expensive on its own but the hardware to mount it its much more complicated and expensive then you think. H channels to fasten them, peak connectors to join at the joins. It needs to be foil taped to stop moisture from getting in. It doesn't work on steel hoop shapes. You will have to build out of pressure treated lumber or steel that has angles instead of curves. Not as easy and cost effective as it seems.