That's an interesting proposition and I wish I could help more with the estimating, but unfortunately I'm already drowning in my own ambition.. I'll throw out a few numbers with the hope that other members may call me out on anything I figured too low.
Anyway, it sounds like you want a box on wheels into which various exhibits, or perhaps DIY learn-by-doing stations, can be installed. It could be a cargo trailer, box truck, or.. gutted school bus. I'm not sure a school bus in inherently better suited for this kind of thing; its construction is so different to conventional US building that I guess you'll have to erect simulated home construction (2x4s, sheathing, drywall, leaky windows) inside the bus shell. If the school bus is just cheaper or more available than a trailer or box truck, then it makes sense (but keep in mind the school bus probably has the lowest ceiling of the three).
For the "empty box on wheels" part of the project:
- a bus. $2-6k.
- catch-up/preventative maintenance, initial repairs, etc.... maybe $1k? This expense is generally bigger when the bus price is smaller.
- labor to gut the bus interior (ie, remove the seats and maybe the plywood and rubber floor). 8-20 relatively unskilled man-hours? Maybe you have volunteer labor available? More time if you want to remove the inner metal skin so that the bus shell can be insulated better.
- heating and cooling for when the bus is open for showing. Maybe $1k each for 1-3 rooftop A/C units plus $4k for a generator to run them, and $2k for a Webasto coolant heater and use the heaters the bus came with. Plus labor costs for installation, if not done free by volunteers. If one of your exhibits involves a normal residential furnace or A/C system, you might actually install an LP-fueled furnace with A/C and ducts instead of the rooftop A/C and Webasto stuff. Still need a decent generator for running the A/C, whether it's rooftop or not.
No idea what you might have in mind for the exhibits.. aren't average US residential construction costs around $100-$150 per square foot now? Hopefully you can bring it in well below there. I guess you'd have to come up with some sketch of the specific construction you wanted and get bids from local builders.
It might be that you've been unable to get anybody to touch it because the bus folks like us aren't prepared to estimate costs for the exhibit portion, and those who could do the exhibit are uncomfortable estimating cost for the vehicle itself.