I think that most commercially 'successful' martial arts schools are either 'on the edge' or are 'subsidized' by other means. The 'pay for the extras', like clubs, special training, etc. is one way to bring more cash in. Running the facility as a gym during the day, or subletting is another. Often, the business is 'subsidized' by students teaching for free, cleaning the dojo for free, etc. And the biggie....defaulted contracts. Referring for a moment to another thread...those defaulted contracts that bring in cash to the dojo...are another subsidy.
As for 'black belt clubs'. Seems to me, that if one has so many students that it becomes appropriate to have a special time for 'bb and above training'...that it shouldn't cost any more. Unless, of course, one is using a space that requires payment for that hour. I think a class of 10 BB's could probably cough up...what $40??...between them for an hour of 'BB training'.