Hi Ray,
You have to do it like this,
Assuming you report is starting row 20,
Say in col E you have your material hierarchy and in col F u have budget and col G u have Actual and in col H you want the difference. In the expension range you have AfterRange, against the column expension intersaction use put the formula "=EVRNG(D12)" and in D12 put the formula "=B12-C12".
When you refresh the report the formula will automatically get copied till the last row of expension.
Hope this helps.
Regards,
Kalyan.