There are a few ways to look at what a rare title is worth, and it can be done by ignoring a recent sale which might have been a one off that might be seen as a crazy price.

Most great games, like brand new Sterns to use as a example, usually sell in Australia for say $6500. (lets use Pro models just for this example cause Pro models have many more years of resale figuresw than the current LE stock).
So a PRO model anything is say $6500 brand new, And for resale the models can fetch between $4500 and $5500 most of the time based on condition, home use only, being minty etc etc.

So, the price drop from brand new, from a percentage point can be calculted in general terms as apx 30% depreciation/loss to a figure of 15%.

Then u get a game like BBB which lets say sold brand new for lets say $10,000 (10 grand in todays money just to make the figure easy to play with). BBB production was apx 200 machines only. The machine is a beauty, it plays like it looks and people love it. It cant be easily replaced, and it has come to represent the pinnacle of A class collectable pinball machine to high end collectors because it is rare, popular and looks the goods.

So i would argue that it is worth the money it was when brand new, cause all low production numbers of anything hold their value and depreciate very little. So lets say it hasnt depreciated the 15 to 30 percent a Stern has/does etc. Therefore, lets say its still worth the $10,000 for arguments sake.
Then u have a buyer who wants one, nobody is selling, and u want one. The seller says I am not selling it unless i get my 10 grand back, and i want profit cause it will never be available again, because they are so rare and nobody sells them. What figure would u place on top, an extra 25 % of original cost or an extra 50% of cost.

Therefore, I would calculate that the machine would be a minimum of say $10,000 to buy, or a maximum of say $15 grand, if using the example money of say $10,000 purchase cost in todays money. That sort of seems maybe a realistic way of trying to work out a price for something rare and extremely hard to replace.

Will it drop in value a bit here n there over the next 5 to 10 years, yes, but it depends on what new machines come out, and just how good they are technology wise compared to the nice games like BBB.  Is it worth 30 grand, no way, could it be argued at $15,000 probably. Could it be argued its worth 10 grand, definately.
Thats how i think most might, MIGHT try n work out a realistic resale price in Australia to maybe most aussies. Just a stab in the dark trying to use a little bit of common sense, if there is such a thing when pricing pinball machines.lol