I doubt they can ever drop, simply because no one are added anymore to the game. They can only disappear (ban, dying, alchimy, ...) So, as they are the rarest in rsc, there will always be a demand by people insanely rich and willing to show it.( less certain with half jug of wine as you cant show it) Therefore, it can only be a good investment over time. It can suffer of some seasonality, but on the long term, it can only increase.