The set-up cost to build "The Big Bus" vehicle was around $250,000. This comes out to around one million dollars in 2022 adjusted for inflation.
The actual bus used for this movie had been part of the Los Angeles Bicentennial Parade in 1976 according to production designer Joel Schiller's personal website.
According to an article in the now discontinued bus magazine "Bus World", in 1976, this movie's actual big bus made a real-life trip in California from Los Angeles to San Diego to promote this movie.
For the big arrival of the big bus, a bus station in downtown Los Angeles had to have its roof raised, so the gigantic coach could enter the facility.
The Big Bus was actually two separate vehicles that could be driven separately (as seen before the closing credits).