Considering the made tens of thousands of magazines and only have a dozen or two of the devices intact, the mag is probably a $100-150 dollar part. Put it on ebay or gunbroker and report back with results.

I owned a semi-auto BAR. When they magazine ban hit from 1994-2004 I saw BAR mags going for up to $250 bucks from various dreamers. Considering there were hundreds of thousands of mags available and probably 1,500 guns in circulation. A decent mag was typically about $20-25 bucks.