shoping cart in asp.net c#

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ASP.NET

Javascript

C#3.5

product Name---in Label
Price ------in Label
quantity-----in TextBox
total cost and Grand total -----in label control

First Task

product name  price  quantity  total cost
a               30     2          60 

product name  price  quantity  total cost
b              20     1          20

Grand Total : 80

here when we enter quantity 2 then ---immediatly total cost and grand total will calculate

Second Task

product name   price  quantity  total cost
a               30     --          0

product name  price  quantity  total cost
b              20     0         0

Grand Total : 0

here when quantity is nothing (empty) or contain 0 then total cost and grand total will calculate 0

above first task i do properly but second task have problem please help

my code

<script type="text/javascript">
    $(function () {
        $("[id*=txtQuantity]").val("0");
    });
    $("[id*=txtQuantity]").live("change", function () {
        if (isNaN(parseInt($(this).val()))) {
            $(this).val('0');
        } else {
            $(this).val(parseInt($(this).val()).toString());
        }
    });
    $("[id*=txtQuantity]").live("keyup", function () {
        if (!jQuery.trim($(this).val()) == '') {
            if (!isNaN(parseFloat($(this).val()))) {
                var row = $(this).closest("tr");
                $("[id*=lblTotal]", row).html(parseFloat($(".price", row).html()) * parseFloat($(this).val()));
            }
        } else {
            $(this).val();

            
        }
        var grandTotal = 0;
        $("[id*=lblTotal]").each(function () {
            grandTotal = grandTotal + parseFloat($(this).html());
        });
        $("[id*=lblGrandTotal]").html(grandTotal.toString());
    }
);
</script>

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false">
    <Columns>
        <asp:BoundField DataField="Product_Name" HeaderText="Item" />
        <asp:BoundField DataField="Unit_Price" HeaderText="Price" ItemStyle-CssClass="price" />
        <asp:TemplateField HeaderText = "Quantity">
            <ItemTemplate>
                <asp:TextBox ID="txtQuantity" runat="server"></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText = "Total">
            <ItemTemplate>
                <asp:Label ID="lblTotal" runat="server" Text="0"></asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
    </Columns>
</asp:GridView>
Grand Total:
<asp:Label ID="lblGrandTotal" runat="server" Text="0"></asp:Label>

private void FillGrid()
  {
    SqlConnection con=new SqlConnection(StrConnection);
       con.Open();
      DataTable dt = new DataTable();
       SqlCommand cmd=new SqlCommand("SP_GetProductDetail",con);
       cmd.CommandType=CommandType.StoredProcedure;

       SqlDataAdapter da=new SqlDataAdapter(cmd);
       da.Fill(dt);

      GridView1.DataSource = dt;
      GridView1.DataBind();
  }