for (int i=0; i<dgKisiFatura.Rows.Count-1; i++)
    for (int j = 3; j < dgKisiFatura.Columns.Count; j++)
           {

           dgKisiFatura.Rows[i].Cells[j].Value = string.Format("{0:n2}", Math.Round(double.Parse(dgKisiFatura.Rows[i].Cells[j].Value.ToString()), 2));
           dgKisiFatura.Columns[j].DefaultCellStyle.Font = fontSayi;

           }