In our area, school buses go for about $2500 and are "used and abused." The owners do not replace them until state and federal funding has expired. Keep in mind that most are geared for city driving, not running down the interstate. Gas mileage is pretty poor also.
But if you get a good one, and block the extra windows with sheet metal before you paint you can get a cheap potential RV. They usually are maintained well and if you can talk to a school district person, you might find out about one that just had the transmission replaced before it had to be sold, for example. They vary greatly in condition and newness of various parts. I've seen them do engine jobs on a bus and then sell it within 5,000 miles. All this means local school districts instead of eBay.