I have heard that they charge $25 to ride to the top on Friday and Saturday night, so it is good to keep this in mind if you are planning to do this over the weekend. Another option is going to the top of the Bonaventure hotel, which is free and has a revolving bar/lounge.
(*HINT: Take the metrolink or subway system downtown and enjoy a day in the city. Your metrolink ticket allows you to use the subway system for free!)