Weighted Average in Grand totals of the pivot table

[mauritzp]

Hi,
I am trying to calculate a weighted average in the Grand Total of a pivot table.
I have lots of columns and one column carries volume which I need to use as the weight, so I am trying to show the Weighted average of all the different types of cost of the items grouped together. can somebody please help as I cannot find a way to do it within the pivot table.

any suggestions will be appreciated.
thanks