If you don't have a basis what the price should be, you can't be sure it's a bubble. Not having a basis for the price doesn't mean something is a bubble, but it could be a red flag. The fair market value is generally what the market decides is the price for it, and there is currently a lot of support for both the price of Bitcoin and Bitcoin Cash. But fair market value is not the same as inherent value, and I'm not sure that either BTC or BCH has any inherent value. So where there is a huge disparity between inherent value and market value, I would be worried that something arbitrary can crash the price for no reason and without warning.