Question: Since full autos are legal to own by civilians under the 1934 NFA, why are they so very expensive to buy?
Answer: In 1986, Congress passed legislation that forbid new production of full auto machine guns for civilian sales. Sales could be made to the military, law enforcement, and possessed by NFA dealers as "dealer samples." A Dealer Sample is a new or used full auto that is used in the dealer's business as a demonstrator for sales purposes. If he wants to sell it, it goes to either another NFA dealer, law enforcement, or to the military -- but civilian sale is prohibited.
Result: The numbers of full auto, transferrable guns was artificially capped and all of these guns were grandfathered -- that is, they could be sold to individuals.
Supply and demand: (1) fixed number of transferrable guns with no possibility of new guns added to the pool; (2) a demand for full auto guns fairly constant; (3) price on transferrable guns goes up, up, up, up. The result is that the transferrable full auto gun's prices are artificially increased due to a shrinking pool of guns (full autos wear out and break and this reduces the overall number available). The logical solution would be to return to a pre-1986 situation and allow new manufacture and civilian sales. This would be a tremendous shot in the arm for all sectors of the military, law enforcement, and civilian communities because the shackles of the 1986 legislation have severely hampered development of select-fire weapons. How likely is it happen? NASA will get to Mars first.