No one has done it so I will...
On May 19, 1986 the making or registering of new machine guns was BANNED! Forever fixing the number of transferable guns available, since that time prices have continued to rise.
The BIG jump in prices began with the internet, as more and more people became aware of legal machine gun ownership in most states, and demand increased as the supply remained fixed.
Prior to the internet the class 3 community was a lot smaller than it is today, and it seemed most wanted to keep it that way. In 1965 there were a reported THREE class III dealers in the U.S. One of them was the (in)famous Curtis Earl, he is the one (I think) that has reported that number.
The only positive event in recent times has been the elimination of the LE signature, which had been impossible for many to obtain, simply because of the anti-gun opinion of the LE officer in their jurisdiction.
Good Old Days SAR article of $500.00 M16s and $73.00 S&W Model 76 subguns!
Interesting Curtis Earl SAR article