Fully automatic firearms, or "machine guns," have been controlled since the National Firearms Act of 1934 was passed in 1934. This created a registry and a tax system required to own them. You must be in a state that allows them, Washington state for instance bans them unless you owned it before 1995, then you must submit a "Form 4," Application for tax paid transfer of a NFA firearm. Fingerprints, pictures, and questionnaire along with the payment of $200 are submitted to the BATFE's NFA branch. Currently the waiting time is about 12 months for approval.
Additionally, because of a rider to the Firearm Owner's Protection Act of 1985 no machine gun made after 1985 (might be May of 1986 actually) can be transferred to anyone except divisions and subdivisions of the US government and certain types of dealers. The military, police departments, government security agencies, but not many tribal police because of the their semi-sovereign status, unless they are deputized/commissioned by the Bureau of Indian Affairs or another law enforcement agency.
Because of the FOPA, the number of full auto available can only go down as they wear out or are damaged beyond repair. The least expensive full auto firearm is something like MAC-10 that starts at about $5000 USD. The most expensive I can think of is the ONLY transferable M249 Squad Automatic Weapon/light machine gun. Not a Minimi but a properly marked and configured M249. Dan Shea used to have it. I believe it sold for $550,000 USD the last time it sold.
M134 mini-guns (powered gatling guns) are a bit over $100,000.
More reasonable priced guns are around $15-20,000 for M16s. Heckler & Koch are quite a bit more, and transferable M60s are around $60,000 USD.
Full auto requires a lot of patience and a lot of money.
Source of knowledge on this is working for a Type 07 FFL (manufacturer, though we don't make much right now) with a Class 2 SOT (Special Occupational Tax, a Type 2 is a maker of NFA items, a Class 3... where the mistaken term of class 3 license, or class 3 firearms comes from... is a dealer of NFA firearms). And having worked for another previous to this. I have assembled/manufactured/made NFA firearms. I've spent a lot of hours reading the BATFE's regulations and looking up the laws... and there is still a LOT I don't know. I know enough to know that I don't know a lot.