Yep. They have been highly regulated. Only a class 3 license in certain states will grant you the capability to purchase an automatic firearm, and on condition the weapon must have been made before 1986 AND does not contain a certain amount of extra parts (i forget how many... i think its 5). There are also huge fees, taxes, and extensive background checks/interviews/courses involved.
Due to these restrictions, the weapons cost a fortune. If you got a license and are an arizona resident... an M16 would cost you over $10k due to the rarity of legal to buy models.
http://www.thefiringline.com/forums/forumdisplay.php?f=4
Well.... You can also pay $200 for the conversion parts. 10 years of prison assrape and $10,000 fine when the ATF finds out.