All pins have gone up in price, including cheap and/or shitty ones. I don't understand the great mystery here. To pinball collectors and players, all machines have been in some demand to some degree for some price, and the market will dictate what those prices are. If something doesn't sell, the owners will either drop the price or decide they would rather keep it. Lots of factors dictating how much demand in the market, whether it be covid buyers, first time buyers, etc or new restoration materials available or someone putting out new code for the game.
If you really think about it, there are a lot of reasons why prices and desirability of certain titles would go up, but there isn't many reasons why they would go down.