I just watched an auction online yesterday that occurred in Tempe, AZ. They had a handful of MiniMills (aged 2012 and older - I think there was a redesign in 2013). The two newest machines went for $20k+18% buyer's premium. These were sold on an as-is basis as is the case with most auctions. The older machines with the older controls went for $10-13k.
My thoughts after watching a bunch of these auctions is that $20k for a 2012 MiniMill in 'as-is' condition was a little steep, but auction prices can vary easily by 20% for a variety of reasons which are sometimes unrelated to the machine's options, age, and condition. Not quite what you asked, but hopefully this is somewhat useful.