The containers aren't cheap either. I recall seeing some for sale for a few grand and that was a private sale. The container companies seem to be very conservative about showing their prices on their sites, so I can only guess it's very expensive probably like over 5 grand. There's a neat appeal to it but honestly, at that price, it's probably not any more expensive to use plywood. You still need 2x4 studs inside, you still need insulation, and you still need a vapour barrier, and drywall. So really all you're saving is the outside sheathing material, and I guess siding etc, since you'd probably keep the metal part as the outside wall.
One main appeal of containers is you could get the skeleton of a building up fairly quickly though so you can work on everything inside without worrying about the rain etc.