In addition to supply and demand (little supply, much demand) there is a $200 transfer tax for every sale between private parties, between a private party and a dealer and between a dealer and a private party. This adds up after a few sales.
Actually, there are not a lot of full auto AK47s in the country, and almost all of them were converted legally here prior to 1986. Trade regulations with China and Russia plus a general ban on importing ex-military arms and full auto arms for private sale prevented importation of the selective fire AKs. Of course, there are thousands of semi-auto AKs, but that is not the subject here.