Auction prices vary widely even with simple items (like a vise or a ceramic plate). On eBay, somewhat unlike advertised physical auctions, they depend not only on who wants whats for sale that week but also on whether someone happens to see it that week. If only one person who really wanted it happened to see it that week, he might have gotten a real bargain.
When you get to the typical lathe being sold by the current owner or by an equipment consignment seller, they more often than not include a widely varying number and value of accessories and from a little to a lot of tooling. There is never any way to say what the bare lathe itself accounts for out of the selling price. It's my opinion that even if you had access to several years of eBay and Craig's List and other sales information, you couldn't compile an anywhere near accurate "blue book" for any make of lathe.
Robert D.