These were made after WWI from converted Gew98's to help stimulate the German economy.
The smaller guages (16 and 20) are actually better shotguns to have, as the 12's required removing so much material from the receivers, that it made them relatively weak, thereby limiting the use of higher power loads in them. The 16's and 20's are more desireable, and generally harder to find. Is your functional and complete?
The 12's usually go around $150-$225 or so, depending on condition.