I bought a 20' "watertight condition" for $1200 delivered last year. A 40' was $400 more. It has a couple small dents, but no breaks in the skin. They had aluminimum ones available too, but more costly (sorry, but I don't remember that price).
Things to consider:
They get really hot when sealed up and sitting in the sun. I used to store my tractor there before the house was built but had great concerns about the high temp. and the onboard fuel. Same would go for any fuel-powered tools, oils, etc.
Have them place it where you want it. Mine came on a rollback truck and they couldn't get it where I wanted. I tried several techniques to move it, but resorted to hiring a D6 dozer to pull and push it into position (1/8 mile on hilly dirt roads). My
B7800's loader won't even lift a corner, and the cross beams on the bottom makes it hard to slide.
As mentioned, the doors on the narrow end make storage planning important.
All in all... I would do it again. I plan on pushing it back into a hillside and covering half of it with dirt and the other half with stone to make it a more permanent addition of my property now that the home is completed.