Each real estate professionals office will have their own policies and fee's. These would be disclosed to you up front. Depending on the office and the apartment/landlord involved you might be charged up to 1 months rent and re-keying fee. Also if the landlord is paying the fee or part of it your costs could be less. There are quite a few great rental real estate professionals here on Trulia. You should interview with a few and see what they have to say. Hope that helps.