Even prior to the 1986 freeze, the price on (private citizen owned) FA guns was always in a climb, due to the fact that $200 was paid as tax each time the gun was sold. And back then, $200 was still real money.
An FA gun with a selling price of $500, transferred through 5 owners over a few years earns the Fed $1000 in taxes. And owner #6 has to pay at least $1,500 to #5 and $200 to the Fed.
With the closing of the civilian registry in 1986, that steady climb skyrocketed. The intended result is, obviously, to eventually end legal private ownership of full auto firearms, as with the supply fixed, by law, eventually they will wear out, or become so valuable as to never be fired.
The law has created 3 price levels for full auto guns. Take a classic example, the Tommygun (1921 or 1928 model with all the trimmings, for example), 3 guns, identical, except for their legal status.
Gun#1) legal to sell only to the police or military, worth only a few hundred dollars, because the police & military don't want Tommyguns much anymore.
Gun#2) never registered, and cannot be legally registered today. Worth 10 years in jail, a $10K(+) fine, Fed. felony conviction ( and all that goes with it) to any one caught with it.
Gun #3) legally registered and can be transferred to civilian ownership. Gun is worth what the market will bear, as there are only a few, and no more can be made. $20-40,000 and up. Kind of like rare old wines and paintings, the only way to get one is to find one somebody is willing to sell, and pay whatever they ask for it.