Less rare. I've gotten half a dozen purples and haven't seen a single white-black. The fact is: the OP wants something very rare for cosmetic reasons. Everyone wants this crystal for cosmetic reasons. It has a very high demand and a very low supply, therefore ppl can charge pretty much whatever they want for them. I've seen the purple and cyan ones for 100-300k, which is cheaper than most end game mods. (because these crystals can be used all the way up to lvl 50 and beyond. Once 2.0 comes out there will be better ones, of course.
So they do have a reason for charging such a high price for the black-white crystals.
Its not like all the people on the server want to rip you off. Or maybe they do on your server