one aspect I see leading to price increase in older titles is REPLACEMENT COST.

I cant speak for each specific part/item, but its possible that just about everything that goes into building a pinball is up from the 80s/90s, especially LABOR. you simply can not build these older ss, an, dmd pins from scratch for less than where the market is heading. from a sum of the parts/labor standpoint, prices on 80s/90s games may still have some catching up to do.

I believe gary stern said he will not be building remakes of titles like star gazer, quicksilver, and seawitch because he doesn't believe people will pay 5K+ for them. upon reading comments like that, one can grasp what it would cost to replace/recreate those titles from scratch and why the market on those and similar era titles are getting bid up.

during the housing crisis, homes in foreclosure/short sales were being bought up, oftentimes by huge consortiums that realized they were getting homes for less than they could be built. those were 'deals', just like pinball finds of the past. as those below replacement cost homes were absorbed, it eventually brought the remaining inventory up to a market that was more in line with actual replacement cost.