What material is the best? That depends on your situation. And prices are difficult to compare, because each design provides a different amount of growing space and has a different lifespan.

While glass is still the best for frost protection and light transmission, and its weight helps to keep it in place in high winds, it is expensive and you may have other things to consider such as children's safety or likely vandalism.

Twin-walled polycarbonate sheeting is a good substitute and is used commercially because of its additional advantage of resistance to damage.

Plastic sheeting, if UV stabilised for longevity, is also a useful frame covering, although not as long lasting as the other materials, and it does mean that sides will roll up for easy access. Polythene cannot keep the frost out as well as other materials but you don't pay much for it, so it's useful if you're on a tight budget. You can still grow a lot of plants under polythene, especially if you insulate the structure at night with some garden fleece or a home-made thermal blanket using polystyrene beads.