Of course, the case of Beanie Babies is more comparable to Dogecoin than Bitcoin, and the Dutch tulip story has in reality been quite significantly overblown (see http://en.wikipedia.org/wiki/Tulip_mania#Modern_views , scrolling down to "Legal Changes"). But then I suppose the reference class of "highly unique things" will necessarily include things each of which has unique properties... :)

I think the way to go here is to assemble a larger set of potentially comparable cases. If you keep finding yourself citing different idiosyncratic distinctions (e.g. Bitcoin was the only member to be not-overblown AND have a hard cap on its supply AND get over 3B market cap AND ...), this suggests that you need to be more inclusive about your reference class in order to get a good estimate.