Recently battled with the notion of dropping an insane amount on a MM.
My AC/DC Premium cost me just over 7k taxes in and it's unbelievable.
I have a tough time believing that an MM at twice the price will bring that much more fun to my house when it's much older technology. Sure, it's a great game and rated number 1 for a good reason, but with any kind of technology, newer has some benefits.
The LEDs in my AC/DC and the programming kick serious butt.
I expect 15-20 yrs from now, nobody's going to want AC/DC if AC/DC III has come out.
I need to log some hours on an MM to see what all the hype is about.

I would expect that if the Wizard of Oz and Hobbit take off, people will see what can be had for $8000. At that point, will there still be a market for older titles at 15-20k? Technology may solve this pricing issue once and for all. Anyone paying much for an EM machine?

The one great thing about pin prices is that they haven't popped yet. With any luck, you can buy a great pin, play it til you get bored, then sell it for slightly less than you paid, just as much, or if you're lucky -even more!

I think another reason these things are going way up in value is the mod factor. Even though a new MM or AFM sold for 3-4k originally, think of what you'd have to pay to bring it back to that condition even if you had one given to you for free. New PF: 1200-1500, plastics, cliffy's, LEDs, mods, decals... you can spend a few grand depending on the game -and that's doing all the work yourself if you're able.
Essentially what we're seeing is that the value of these things is more in line with the cost of shopping them/upgrading them than the original cost. So long as there is a finite number of people who can shop a pin to mint condition, and we're keeping those people busy beyond what they can handle, they're not dropping their prices for any reason. A mint game now becomes way more expensive, and the routed machine with the same name also goes up in value.