I think a lot more energy goes into producing steel. I bet you would find a keto type product is cheaper vs steel in the states where they seem to use timber products a lot more
I think there might be 2 kinds of spray or that could just be bull. I've had it before when someone has said ive put the good spray on for you in case tiling the floor or something. Maybe acrylic vs non acrylic?
1 section. Its quite common to have a hole like this somewhere. On some sites they cut a hole 1400 by about 200 to pass plasterboard up through then nog and patch after.