It’s not your imagination! I’ve only been really following the hobby for a year and a half or so and the prices have gone up even in that small time frame. New games are more expensive than ever and it seems like the popularity of pinball is increasing, so I think this is part of what you’re seeing. Also, economy is fairly robust and people have more money to spend. I would guess that most games are selling for more than the pinside prices that are listed. Perhaps archived prices should be removed after a certain amount of time from the price formula so that they are more accurate.
Good news is that more money in the hobby= more games coming out.