So here are some interesting statistics:
According to IPDB, about 1.1 million pinball machines have been produced since 1980. I don't think that includes Stern's numbers, since they don't publish any. A lot of machines are long gone, but let's be generous and say Stern has made up the difference and there are still a million Solid State machines in circulation.
According to Pew Research, about 16% of the world's 7 billion population is upper-middle or upper-income. These would be the target market for luxury goods such as collectible pinball machines. That's over a billion people.
So there's roughly one solid-state pinball machine per 1,000 people worldwide in the target collector market. That looks like a deficit to me. I think the price increases are pure supply-and-demand: Demand is going up and suppliers can't produce more yet.