Definition of a "Machine Gun" is any firearm capable of firing more than a single shot with one pull on the trigger.
Legal MP-5's run about 12K-14K these days.
If you are in any of the states that allow them they are not that hard to find.
Most MG owners start out with a less expensive gun, MAC-10's run about $2,500 these days, STEN's around $3,500, etc.
A G.E. Mini-Gun is about 250K without accessories, just in case you are rich!
Was thinking more along the lines of a Licensed HK knock off...Tactical LLC, Bobcat etc...