The modern German issue gravity knives are made by Eickhorn of solingen (who makes most issue cutlery, except for the small utility pocket knife which use to be made by victorinox, and now the Italians are making it, still asking myself why????).
anyway when I was stationed in Frankfurt Germany (Rheinmain AB) in the late 80s, early 90s you could buy one of thes for about thirty dollars. you have to be careful as there are alot of knockoffs out there. They are an interesting conversation piece, but there not really much of a good use otherwise. They were made to sever parachute lines, and not to be a "real survival tool". The Materials and construction are not really worth the money you could end up spending to get one now.
However if you want to have one just to have it, its not that bad (just remeber theres not alot of uses for it).
If you can find the older ones with wood slabs they would be worth some money, but buyer beware they are often copied!!
hope this helps!!