Grouping/Appending GridView Header

GridView header to have a Parent Header(Grouped Header) is as simple as the following.







protected void GrvPrePromotion_RowCreated(object sender, GridViewRowEventArgs e)
    {
        if (e.Row.RowType == DataControlRowType.Header)
        {
            GridView HeaderGrid = (GridView)sender;
            GridViewRow HeaderGridRow = new GridViewRow(0, 0, DataControlRowType.Header, DataControlRowState.Insert);
            TableCell HeaderCell = new TableCell();
            HeaderCell.Text = "Promotion Product Group";
            HeaderCell.ColumnSpan = 3;
            HeaderGridRow.Cells.Add(HeaderCell);
            HeaderGridRow.CssClass = "gridheadtop";

            HeaderCell = new TableCell();
            HeaderCell.Text = "Sales and SGM";
            HeaderCell.ColumnSpan = 4;
            HeaderGridRow.Cells.Add(HeaderCell);

            HeaderCell = new TableCell();
            HeaderCell.Text = "Special Discount";
            HeaderCell.ColumnSpan = 3;
            HeaderGridRow.Cells.Add(HeaderCell);

            HeaderCell = new TableCell();
            HeaderCell.Text = "Rebate";
            HeaderCell.ColumnSpan = 1;
            HeaderGridRow.Cells.Add(HeaderCell);

            HeaderCell = new TableCell();
            HeaderCell.Text = "Free Goods / Prize";
            HeaderCell.ColumnSpan = 1;
            HeaderGridRow.Cells.Add(HeaderCell);

            GrvPrePromotion.Controls[0].Controls.AddAt(0, HeaderGridRow);
        }
    }

 How to Display/Calculate Total/Percentage in the Gridview footer.