I haven't kept track, but a report in the middle to late 1980s, after the law banning civilian acquisition of any machine gun made after the Sep 15, 1986 (?) cutoff date, said the MG population was around 220,000.
So everything is gonna go up, if the economy keeps going well.
I haven't priced one lately, but the good old "Tommy Gun" would be my guess as an investment. Relatively rare, and has a certain sentimental value...Plus, if you have a progressive reloader, it's cheap and easy to keep it well fed.