I believe the price is mostly just a factor supply and demand.
AFM production is pretty low - 4,200.
Compare that to other popular pins:
TAF - 20,228.
So ~4.8 more TAFs than AFMs exist. So if a TAF is worth $4K worth of fun, it isn't unreasonable to believe that AFM could be worth ~$20K.
Notice that AFM would be higher in price in this scenario. I think more people like TAF than AFM, so that's why the ratio just based upon production numbers doesn't hold up.
MM has a production run close to AFM, so if they were in the same demand, we would expect similar prices. So perhaps TAF and MM have about the same demand and are relatively equally liked, but there are just so many more TAFs out there.
I find this an interesting way to look at pricing trends and popularity of pins. It is not very effective on newer pins because the pricing data is limited and production runs less certain.
I looked at this a couple years ago (some of these prices have moved a fair bit) and that the pins in popularity order (based on this approach) were:
TAF
Indiana Jones
Twilight Zone
KISS
Medieval Madness
STTNG
Fish Tales
Funhouse
Terminator 2
As for me, I love AFM. One of the pins I have had for years and don't plan to sell. I played it in the arcade when it came out. I just recently bought a pin from a guy who has been collecting for 40 years. AFM was his favorite DMD. It is a great game. And surprising few people have played all the bits and pieces (Again!).