total cost of list box's multiple selected item_code

protected void btn_Click(object sender, EventArg e)
{
   string lot_code ="";
   foreach(Item item in lstBox.Items)
   { 
      if(item.IsSelected)
        lot_code+="'"+item.SelectedItem.Text+"',";
   }
   lot_code.TrimEnd(",");

   //Execute below query to get the result from the database
   //"Select sum(cost) from tableName where lot_code IN ("+lot_code+");"
}

int sum = 0;
foreach (object item in myListBox.SelectedItems)
{
  DataRow [] dr =myDataTable.select("lot_code = " int.Parse(item.tostring()));
  sum += dr[0]["cost"];
}