I don't know if I could afford to own one no matter how cheap the firearm was; sorta like shootin' anything between nickels and quarters downrange at a rate of 300 / min! They are expensive to shoot!
However, in the US Army, some of the most fun I had (aside from launching grenades or shooting LAWs) was firing the M-2HB. Pain in the but to clean, though - heavy springs, and massive assemblies. The reciever alone weighed at least 50 lbs.!
Re; Ammo prices. Supply and demand. Government over regulation is choking certain markets. This is true in more than just the firearms, and related, industries.