addition of data grid column

you can do some in the footer of grid view so you have to do following two step

1. do total and add to footer
int sumFooterValue = 0;
protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
{
  if (e.Row.RowType == DataControlRowType.DataRow)
  {
    sumFooterValue += Convert.ToInt32(e.Row.Cells[1].Text );
  }
  if (e.Row.RowType == DataControlRowType.Footer)
  {
     Label lbl = (Label)e.Row.FindControl("lblTotal");
     lbl.Text = sumFooterValue.ToString();
  }
}

2. Add Footer template to gridview for this you can also check msdn or google