I've been thinking of having an upper limit to the number of rooms you can have in a brothel. It doesn't seem reasonable that you can take a 20 room brothel and expand it by another 280 rooms, or whatever, so I was going to go with double the initial number of rooms.
That would give you a grand total of 400 rooms when you have all the brothels expanded as far as you can.
Would that be enough for most people playing? Would you want it to be an option (set in config.xml)?