Calculations in Microsoft Outlook 2007 Calendar

In the “All Appointments” view of my Outlook 2007 Calendar:

I have inserted an appointment “Duration” field.
I have inserted a “Billable Rate” field.
I have created a calculated field to multiply the numbers in the above fields and am now displaying the “Invoice Amount” for each client appointment.
I have grouped the items by client name.

How can I automatically total these invoice amounts to display the “Grand Total” for each client?