I expect there are many different arrangements, but the incall establishments I've visited have been free to enter. They have an 'owner' who usually employs a maid/receptionist, these provide the premises, the advertising, the bookings, the laundry, the security, all the backroom stuff.
The girls turn up and work, they keep what they earn but hand over a percentage to the house, usually a third. I've heard of others where the girls are charged a daily fixed fee for the use of the room, anything they earn beyond that is theirs to keep.