$60 PLUS the wall coverings, PLUS the insulation, PLUS the cost of the equipment used to weld all these together, not counting electricity or gas to power your welder, PLUS your time.
I think if you can get free cubbie walls, the modifications to retro fit into a bus should be minor vs building from scratch. Haven't pulled the cloth off any, but I would assume that would be minor, which would leave you with just insulating the inside.
Personally, having spent toooooooo many years surrounded by those walls I would not even consider them.
Just my 2 cents, my thoughts on paper. (As I've been told "if you don't know what your talking about....don't talk).