Sum of XML data

I am reading an XML file and writing its contents to a Repeater control using the following code:

 Dim dstTS As DataSet
 dstTS = New DataSet()
 dstTS.ReadXml( MapPath( "cw052004.xml" ) )
 rptTS.DataSource = dstTS
 rptTS.DataBind()

It works fine. However, I would also like to calculate the sum of a the "item_cost" from each node in the XML file and place it in a variable.

Can anybody suggest the best way to go about this?do some searching for examples on the itemdatabound event. this is fired everytime an item is bound and you can use this to sum up your grids and such.
Awesome, thanks!