I had a thought the other day about a potentially big influence on the recent rises in pinball prices (especially NIB games).
A few years ago, after plenty of delays and doubts, Big Bang Bar remakes were delivered. If I remember right, prices on those games immediately skyrocketed and some who took the risk of pre-ordering a game were able to sell their games at double what they paid. As this happened, there were a lot of people kicking themselves for not ordering a BBB in the first place.
Fast forward a few years, and Jack announces that he is starting a new pinball company and making a game. Delivery schedule is tentative, but he's got big dreams and is taking pre-orders. Consciously or not, I think a lot of people had BBB flashbacks with that announcement, and were not going to miss this chance at getting a rare and highly valuable machine. Of course peoples' rapport with Jack and desire for something new were big factors, but I think the "BBB Factor" probably made it way easier to put forward a lot of money on a potential risk.
With so many people learning their lesson from missing BBB, Jack gets plenty of pre-orders. Meanwhile, Stern sees what people are willing to pay for machines, and they up their prices also. And this chain influences the used market prices also.
I'm not suggesting that the economy, growth of the pinball market, greed, etc. aren't factors, but I wonder if BBB is a subtle yet powerful influence here. I guess one way to gauge this would be to survey WOZ "owners" on whether their main motivation to buy was Investment or pure interest in owning the game itself. [feel free to vote in the poll if you are a WOZ owner]