there arent as many like EC says, plus many short busses are handicap units.. they are extremely expensive to buy new as many were equipped with wheelchair lifts, special seating, air ride, and air-conditioning.. sao the schools tend to hang onto them longer than other busses..
many of them command higher prices as they go right back into bus service as church busses, secondary schools, and daycare centers that need to transport handicap individuals..
sometimes you can find non-handicap shorties and they will tend to go for less as they are just a short regular-spec school bus.
also right now with Covid-19 all busses are priced higher on the used market.. as schools reopen many have protocols which limit the number of students per bus so schools are operating every bus they can get a driver for as opposed to selling off what wouldve been spares..
other schools put-off buying new busses as their fleerts sat idle for ??? months so they didnt replace aging busses as the future of in-person schooling was up in the air.
the popularity of social-distanced travelling by RV in general has skyrocketed thus people building RVs and tiny-homers out of busses got more popular. shorty busses are easy to park, easier to stealth camp, they fit in tighter spots thus are often a favorite esp for single people and those looking for short-term weekend-trips.