You can use DataTable.Compute Method to get total amount and use below method to add total row in GridView. Below is parameter description \
gridView: gridview instance
cellIndx : index of column on which you want total
total : total amount.
private void AddTotalRow(GridView gridView, int cellIndx, decimal total)
{
GridViewRow rw = new GridViewRow(-1, -1, DataControlRowType.DataRow, DataControlRowState.Normal);
TableCell rowCell;
int count = 0;
for (int i = 0; i < gridView.Columns.Count; i++)
{
rowCell = new TableCell();
rowCell.Text = "";
rowCell.Style.Add("border-top", "2px solid black");
if (cellIndx == i)
{
rowCell.Text = String.Format("{0:C}", total);
rowCell.HorizontalAlign = HorizontalAlign.Right;
}
rw.Cells.Add(rowCell);
}
gridView.Controls[0].Controls.Add(rw);
}