ทำการคำนวนใน Gridview ที่ selete มาจากฐานข้อมูล ให้แสดงเป็นคอลัมใหม่อย่างไรครับ(C#Winapp)

            opendb();
            SqlCmd.CommandText = "SELECT Or_ID,Returner.PP_ID, Product.PP_Name, Amount, Price, Note FROM Product,Returner WHERE Re_ID='" + tbReID.Text + "'AND Returner.PP_ID=Product.PP_ID ORDER BY Re_ID";
            Sqldr = SqlCmd.ExecuteReader();

            DataTable dtType = new DataTable();
            dtType.Load(Sqldr);
            dgvList.DataSource = dtType;
            Sqldr.Close();

            for (int i = 0; i <= dgvList.Rows.Count; i++)
            {
                SqlCmd.CommandText = "Select Amount,Price from Returner where Or_ID='" + dgvList.Rows[i].Cells[0].Value.ToString() + "' AND PP_ID='" + dgvList.Rows[i].Cells[1].Value.ToString() + "'";
                opendb();
                Sqldr = SqlCmd.ExecuteReader();
                if (Sqldr.HasRows)
                {
                    Sqldr.Read();
                    int AAmount = Sqldr.GetInt32(0);
                    double PPrice = Sqldr.GetDouble(1);
                    //PPrice = Convert.ToInt32(gvSale.Rows[i].Cells[2].Value.ToString());
                    //Convert.ToInt32(PPrice);

                    double total = AAmount * PPrice;
                    total = Convert.ToDouble(dgvList.Rows[i].Cells[6].Value.ToString());
                }
            }

            setgvProduct();