Seems like a lot of confusion about how and how much real fully automatic weapons can legally be purchased. Just a quick primer:
1) Yes you can if your state hasn't banned them. The Feds have a list of registered full auto guns grandfathered into the system that are transferable between members of the public. No license required. Pass the background test and pay the tax stamp and your in.
2) Costs varies. Cheapest things now seem to be Mac10s at about $7-8K. An M16 variant (including other brands and conversions) start at around $18K. And MP5 around $30K.
And of course, if you own one the feeding of it can be expensive.
Owning one includes having a full FBI background check. All of the above is also true to own a suppressor (silencer) or short barrel rifle.