It's usually helpful to look at recent sales: https://pinside.com/pinball/market/classifieds/archive?s=1&ad_machine_key=536
It's hard to find the amount sold for most of them, although it's a good bet that "Firm" listings sold for close to the listed price. You can compare and contrast your machine with the ones listed and see why expensive ones are expensive. IMO if you're patient you might be able to get low $5K range.