Files to look at:
ASPxPivotGrid - How to calculate running totals
This example demonstrates how to include previous cell values in values of the next cell. To calculate cumulative values, set the RunningTotal property of the corresponding field to true.
In this example, enable the Include Previous Quarter Sales into the Values of the Next Quarter check box to include previous quarter sales in the next quarter sales value. The Allow Cross-Group Running Totals Accumulation check box allows you to specify whether running totals are calculated independently within individual groups or for the entire Pivot Grid.
See also: Running Totals