AJAX Postback did not give expected result

$("#<%=registrationNumberTextBox.ClientID%>").blur(function() {
    if ($(this).val().length > 0) {
        $.ajax({
            type: "POST",
            url: "registration.aspx/GetAmountDue",
            data: "{'registrationNumber': '" + $(this).val() + "'}",
            contentType: "application/json; charset=utf-8",
            dataType: "json",
            async: true,
            cache: false,
            success: function(msg) {
                $("#<%=amountDueLiteral.ClientID%>").val(msg.d);
            },
            error: function(x, e) {
                alert("The call to the server side failed. " + x.responseText);
            }
        });
    } else { $("#<%=amountDueLiteral.ClientID%>").html('0.00'); }
});

<webmethod()>; _
Public Shared Function GetAmountDue(ByVal registrationNumber As String) As String
    If registrationNumber.Trim.Length > 0 Then
        If Student.IsValidRegistration(registrationNumber) Then
            Dim bi As Student.BasicInformation = Student.GetStudentByRegistrationNumber(registrationNumber)
            If bi.StudentId <> Nothing Then
                Dim result As String = String.Format("{0:n2}", bi.StudentAccount.Debit)
                Return result
            End If
        End If
    End If
    Return "0.00"
End Function