As much as Crazy says it didn't happen, prices did dip some in 2009. It wasn't a huge dip, but there was one, at least out here in the midwest. And prices for MM, AFM, and other remake possibility titles have gone way down since that started...if you bought a restored version of one of those 5 years ago, you're probably still in the red.
I think the reason prices haven't dropped is that no one has been able to manufacturer games at volume to compete with Stern. Without that, you can't get oversupply which is needed to drop prices. I think a lot of us (including me) thought that all the manufacturers coming online would cause a drop in new game prices but that hasn't happened at all, in fact they've gone up because nobody can manufacturer games affordably. I think the other assumption is that since the older existing collectors stopped buying, it would stop pinflation, but new buyers keep stepping into the market every day. I'm genuinely surprised at the number of new people coming into the market.
I think the general theory is that pinball prices will fall because existing collectors don't want to pay the higher prices and there's oversupply. The problem is that the new wave of collectors that keeps coming in trumps the existing base and keeps there from being oversupply. So it's one of these deals where theoretically it should have already happened but hasn't. According to google trends, pinball interest is at an all time low, but that hasn't been reflected in the market at all. I really think that the wave of video pinball games on the xbox and playstation helped generate a new wave of buyers to some degree, I've heard from a lot of people that's what got them into this.