As BCannell stated, its really just simple supply & demand. In May 1986, Congress passed a law (18 USC 922(o)) which prohibited the manufacture of new machineguns for civilian ownership & possession, thus the number available to buy was fixed and actually constantly declining due to loss, damage/destruction, and unregistered transfers (if a full auto is not transfered in accordance with the law, it becomes contraband and is no longer legal) . As more and more people learned through the internet that machineguns were in fact legal to buy (provided you bought one that was made & registered prior to May 19, 1986) there were more people with more money chasing after a declining number of legal guns. Increasing demand with a declining supply inevitably leads to increasing prices.