I once owned a MAC 11 (along with a factory suppressor), and, frankly, I didn't think much of it.
The only way that paying astronomical prices for such mediocre machine guns makes sense is if you believe in the "greater fool theory," that is, that someone else will come along to pay more than you. In this economy, that's a questionable proposition. Furthermore, I believe that we'll see the Hughes amendment repealed, and the registry opened up, sometime within the next 10 years. If that happens, the bottom will drop out of at least the low end of the MG market.
In terms of both shooting fun and, if it comes to that, combat effectiveness, you would get "more bang for the buck" by buying several semiauto EBR clones, and lots of ammunition, in preference to a single low-end MG.