Pivot table - creating a calculated field

I am trying to create a calculated field without success.

The underlying pivot table data is as follows:

FISC SKU COM VOL UNIT_COST
2008 151 173 155 8.92
2008 200 20 145 0.59
2008 200 90 145 0.49
2008 200 100 145 0.20
2009 151 173 139 8.46
2009 200 20 130 0.56
2009 200 90 130 0.46
2009 200 100 130 0.19

The pivot table is designed as follows:

Data FISC
VOL UNIT_COST
SKU COM 2008 2009 2008 2009
151 173 155 139 8.92 8.46
200 20 145 130 0.59 0.56
90 145 130 0.49 0.46
100 145 130 0.20 0.19

I would like to create a calculated field that does the following calculation:
=(UNIT_COST for 2009 - UNIT_COST for 2008) x VOL for 2009

Can anyone help?

Thanks