Check out http://www.422sales.com
and look at the market reports for previous auctions. Prices vary by condition, but you can get a pretty good idea. Year, engine, transmission, brakes, body, and capacity will all affect the price, so try to find a bus as close to what you're looking at as possible.
Looks nice, but don't spend more than you're comfortable with to get it. I went to a bus auction last weekend hoping to buy, but with the attitude that I was only buying if I found the right bus at the right price, otherwise I'd keep looking. I found 2 that I wanted - at last year's auctions the same year/make/model went for $700-$900. Great buses, no rust, new tires and brakes, good engines. A couple guys got into a bidding war and they sold for $3,500.