Welcome to the world of collecting and hobbies in general. I just got into this hobby less than a year ago. Bought a couple of older machines for under a grand each and been having fun with it.
However I have seen the exact same thing happen in the classic car hobby as well. I have seen the prices back in the late 90s early 2000s go completely through the roof, thanks to brain dead tv shows like Mecum and Barrett Jackson, where people are paying way too much for a car that just isn't worth it.
So people see that foolishness and think....hey my car, or pinball machine must be worth that too. So up goes the price, even though it has been sitting in a barn for 30 years not running and completely rotted out.
Now, the car market has had the bottom dropped out. Young kids don't like them, (mainly cause it's not a cell phone) so they are not buying them, and I figure the same will happen with pinball too.