The best way to determine how much you should charge is to browse other listings to see what other people are charging.
For example if you have a bedroom to rent for a student then go to Off Campus webpage and on the left hand side click on the heading "Student Listings". This will bring you to a new screen that will show a summary of current Apartments, Shared, Room & Board, House, Rooms and Suites for rent. If you click on "Rooms" that will bring up all of the Rooms for rent and you can compare your place with what other people are offering and charging.