Вам необходимо привязать к событию ItemDataBound:
int counter = 0;
void Grid_ItemDataBound(object sender, Telerik.WebControls.GridItemEventArgs e) {
if(e.Item is GridDataItem) {
GridDataItem dataItem = e.Item as GridDataItem;
counter += Convert.ToInt32(dataItem["SomeField"].Text);
}
else if(e.Item is GridFooterItem) {
GridFooterItem footerItem = e.Item as GridFooterItem;
FooterItem["YourFooterColumn"].Text = counter.ToString();
}
}