Your data is coming from a linked file in your My Documents folder called "Task Usage.cub". I can't change anything in the pivot table as it fires an error. You need to either adjust the source data to calculate the cumulative cost properly, or change the way that the pivot table is generated.

To import the database table directly:
In the data tab select "From Other Sources" then select "Microsoft Query"
Next you need to use the OBDC data source set up for your database system, and choose the table you are importing from "Task Usage.cub"

This will put a refreshable query into your file. You can then create a calculated column in that table for the cumulative cost.
Dan