Cylinder

The calculator is misleading - you see when it calculates the are for
any shape it calculates the surface area of the entire figure, i.e. it
ends up counting area which will not be inhabited.
Now you say we can save money by making the cylinder/disk into a torus.
This is true, if you use a torus you will end up saving money. The
height of the cylinder or the outside side of the torus is actually your
floor and to make a torus into a cylinder you decrease its radius to 0
(the distance between the center and the inner edge of the torus, note
not the tube radius, we are actually increasing the tube radius). So you
get more 'height' by converting the same torus into a cylinder but not
more surface area.