That depends on what type of auction and how much 'attention' it gets. A local live bidding only auction tends to have lower prices than an online auction. Online auctions from more 'niche' sites like publicsurplus or govdeals in turn usually have lower prices than ebay. An ebay auction is exposed to the world where any random person may decide to throw some money at an item with a credit card, where as a live auction requires MUCH more planning and cash deposits.
That being said, I'd expect a bus like that to go for between $2000 and $3750. I've seen plenty of buses go lower than that, but with that engine and tranny combo there will be enough attention to at least drive it up to $2000, even if it is a small, local auction. Factors driving the price down are the age and size. Too old to be used for school purposes, and too small for many people's preferences for an RV conversion. Meanwhile its too big for many churches or other entities that are looking to easily shuttle groups of people.
If I had to take a stab at the final bid price, I'd say about $2500-2900. If its in really good condition it could definitely be higher though.
*Edited because I read a key factor wrong!