Reporting Services 2005在表中减去2个值

I have a list of fees being displayed in a table object. I would like to display the difference between 2 of the fees in a text box (not sure if this goes in the table footer or in a group footer). I am able to sum values easy but I don't see how I can subtract values. In this example let's say I would like to show the difference of the License fee and Registration fee (999-333). How can I do this using groups / filters etc? BTW I want to do this at the report level not in a stored procedure! Thanks in advance...I have a list of fees being displayed in a tabl