Normally used audio equipment is 50% of the original value. If it is an uncracked maybe it is worth 60-70%

Not sure there is a market in nOrh speakers where you can say how much anything is worth. In the end, it is worth what you can sell it for.
There were no marble 6.9s, only 7.0s, 9.0s and 4.0s, now 3.0s. The 6.9s were the first Synthetic Marble drums, made from marble powder and resin. Sounds very similar to solid marble.
There are articles on the nOrh website that describe with pictures how all the drums were originally made from Monkeypod wood. They learned that the original process for curing the drums did not work for other climates. They began kiln drying for many weeks. Meanwhile they replaced cracked drums free of charge.