It can't be quite this simple. If your renter is paying $X to you plus $Y to your broker, then their willingness to pay for the apartment was at least $X+Y, and you're leaving at least $Y on the table. In theory there should be a lot of market pressure to shrink Y. So the question becomes, what transaction costs are getting in the way of that? Or maybe, is the $Y actually buying something that's of value to the landlord?