There is a pretty decent article here on them, which reading between the lines explains why the arm may be less popular than better known machine guns and submachine guns. Popularity drives price in a limited market.
“The Reisings had reliability issues in combat—so much so that some units discarded them and used ‘03 Springfields (bolt-action rifles) instead.”
That appears to be the case, Smokin’ Joe:
“Quickly despised by front-line Marines, Lieutenant Colonel Merritt A. Edson, Commander, 1st Marine Raider Battalion, ordered that Reisings be flung into Guadalcanal’s crocodile infested Lunga River, as his troops resorted to reliable bolt-action Springfield rifles.” - http://en.wikipedia.org/wiki/M50_Reising