Calculating total of an expression column

I am trying to calculate the total price for an invoice.

I have the following columns in a tabix:

PurchaseOrderNumber, ItemNo, Quantitiy, ItemDescription, Discount, UnitPrice, Subtotal

The subtotal column (textbox8 in the tabix) takes into consideration the unit price, quantity and the discount and is calculate by:

=(((((SUM(Fields!ItemUnitPrice.Value))/100) * (SUM(Fields!Discount.Value))) - (SUM(Fields!ItemUnitPrice.Value))) * (-1)) * (SUM(Fields!Quantity.Value))

I need to calculate the total of this column. I have tried =SUM(ReportItems!Textbox8.Value)

This brings the error:

The value expression for the textrun 'textbox54.paragraphs[0].textruns[0]' uses an aggregate function on a report item. Aggregate functions can be used only on report items contained in page headers and footers.  

Is there anyway of calculating what I need to without the value been in the footer? If not, would '=SUM(ReportItems!Textbox8.Value)' work in the footer? It doesn't seem obvious how to add one.

If it makes a diffrence I'm using Visual Studio 2010, and am creating a Windows Form Application.

Thanks in advance