There are many different scenarios to which your question can pertain and many different outcomes. Since nothing in real estate is "standard" practice, hence all commissions are negotiable etc, the management fee and the way in which you set up the agent to get paid is completely up to you.
Most agents who want to handle the leasing of your building will either work with a property management company and propose to you for them and their property management company to take over the buildings management. If this is the case, the agent should introduce you to the property management company. After meeting with them, they should ask you specific questions as to what your goals are with the building and how much money, if any, you want to bank every month. They should then produce a proforma for you based on current rents, occupancy rate, etc that will tell you their fee and how much you will most likely make. ((Make sure to ask for best and worst case scenario proformas))
If have a superintendent or a property management company and want to enlist one broker to exclusively handle your property, there are many options. I'd recommend that you give the broker an exclusive, let them collect their own fee from the apartments. At my company, if you gave me the exclusive this would cost you nothing. Another option, if you wanted to rent apartments quickly and surely, you could offer the agent a commission equal to 1 - 1.5 months rent of the apartment they just rented (this would be of a cost to you). But in both cases, the marketing expenses should come from the agent - not your own pocket.
I hope this helps! If you would like to know more, please reach out!
CitySites Real Estate Group
(347) 601 - 9600