Add AVERAGE column to Pivot Table
 

I have a pivot table with sales reps for rows and months for columns. The
data is the sum (total sales) for each rep. I have Grand Totals turned on for
both columns and rows. However, I would like to see Total Sum for the column
grand totals and Total Average for the row grand totals (an average of the 12
months). How do I do this? If I go into field settings on my (total sales)
data item and change it to average, both column and row grand totals change.
How can I configure them independently? [Excel 2003] Thanks.