Mbanghart
It seems to depend on whether one wants one or not. According to our host's book "Kimberley Siege Account and Medal Roll", the official rolls show the number of medals issued with the DofK clasp number 5,458. Many were single clasp issues and many had multiple clasps. Recipients included British soldiers and Town Guard, etc. So it seems it is not particularly rare. However, single clasp QSAs have a following all of their own and those bearing the DofK clasp are associated with the unofficial Mayor's Kimberley Star. Which is a particularly attractive item.
So, one pays your money out for your choice - or not. I am not up with the current asking prices for DofK QSAs - but I suspect it reflects the current economic situation.
Regards
IL.