I've gotten one for free, and my current bus was $2150 on GovDeals 18 months ago. Its got a DT466, Allison 643, Air ride, ABS, Auto slack adjusters, underside storage, and got me 800 miles home to FL from KY with no problems. No real rust to speak of either. You just gotta know what to watch for and strike while the iron is hot.
$2500 is about as much as I'd personally spend on a school bus, unless it were really something special.