Javascript 'getelementbyid("<%=lblPrice.ClientID%>") error

Question

I have below code

<script type="text/javascript">

    $(document).ready(function () {

        $("#ddlPackName").change(function () {

            // Your requirment

            sum();

            

        });

    });

    function sum() {

        debugger;

        //               var final;

        var amountToPay = document.getElementById('<%=lblPrice.ClientID%>').value        /*Here throwing Microsoft JScript runtime error: Object required*/

        var noOfConn = document.getElementById('<%=txtNoOfConn.ClientID%>').value;

        var chargePerConn = document.getElementById('<%=txtChargePerConn.ClientID%>').value;

        var installCharge = document.getElementById('<%=txtInstallCharge.ClientID%>').value;

        var discount = document.getElementById('<%=txtDiscount.ClientID%>').value;

        var packPrice;

        if (isNaN(amountToPay)) {

            amountToPay = 0;

        }

        if (isNaN(noOfConn)) {

            noOfConn = 0;

        }

        if (isNaN(chargePerConn)) {

            chargePerConn = 0;

        }

        if (isNaN(installCharge)) {

            installCharge = 0;

        }

        if (isNaN(discount)) {

            discount = 0;

        }

        if (document.getElementById('<%=ddlConType.ClientID%>').selectedIndex == "1") {

            if (document.getElementById('<%=ddlPackName.ClientID%>').selectedIndex != "0") {

                packPrice = amountToPay;

                packPrice = amountToPay + (noOfConn * chargePerConn);

                packPrice = packPrice + installCharge;

                packPrice = packPrice - discount;

                if (!isNaN(packPrice)) {

                    document.getElementById('<%=txtAmntToPay.ClientID%>').value = packPrice;

                }

            }

            else if (document.getElementById('<%=ddlPackName.ClientID%>').selectedIndex == "0") {

                packPrice = noOfConn * chargePerConn;

                packPrice = packPrice + installCharge;

                packPrice = packPrice - discount;

                if (!isNaN(packPrice)) {

                    document.getElementById('<%=txtAmntToPay.ClientID%>').value = packPrice;

                }

            }

        }

       

    }

           </script>--%>

<asp:dropdownlist ID="ddlConType" runat="server" TabIndex="7">

            <asp:ListItem Selected="True" Value="0">SELECT</asp:ListItem>

            <asp:ListItem Value="1">Post Paid</asp:ListItem>

            <asp:ListItem Value="2">Pre Paid</asp:ListItem>

            </asp:dropdownlist>

<asp:dropdownlist ID="ddlPackName" runat="server" AutoPostBack="True" 

                TabIndex="11" onselectedindexchanged="ddlPackName_SelectedIndexChanged">

                <asp:ListItem Value="0">Select</asp:ListItem>

            </asp:dropdownlist>
<asp:TextBox ID="txtNoOfConn" runat="server" TabIndex="15" onkeypress="sum();">0</asp:TextBox>
<asp:TextBox ID="txtChargePerConn" runat="server" TabIndex="17" onkeypress="sum();">0</asp:TextBox>
<asp:TextBox ID="txtInstallCharge" runat="server" TabIndex="19" onKeypress="sum();">0</asp:TextBox>
<asp:TextBox ID="txtDiscount" runat="server" TabIndex="21" onKeypress="sum();">0</asp:TextBox>
<asp:TextBox ID="txtAmntToPay" runat="server" ReadOnly="True" TabIndex="23">0</asp:TextBox>

/*Microsoft JScript runtime error: Object required*/