So, I did an experiment. Took everybody out of the brothel, and operated just my monster-breeding operation.
2 goblin breeding rooms, 2 orc breeding rooms, 2 werewolf breeding rooms, one minotaur breeding room. And also a salamander breeding room, which gave me 2 salamanders to sell and 2 to deliver.
All the monsters bred for maximum conception chance and minimum sex time. Slaves being descendants of special sheep (for maximum automation) or special dwarf (legacy).
My revenues were 20802 gold that day, with ~1100 spent on monster and slave maintenance (most of them being unproductive, with the breeding operation run on slaves and monsters with maintenance of 5 gold).
That is enough to win the game without relying on brothel money. In that playthrough (on Normal difficulty, while keeping a minimalistic brothel for breeding purposes) I had 580 000+ gold in the end. Even after all the upgrades done to please Barbara the Blacksmith.
How much money are people here usually earning with their brothels?