Hi,why not your code use jQuery completely ?
try this
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script type="text/javascript">
$(function(){
$("#btnCalculate").click(function(){
var consultFee = $('[id$=txtConsultFee]').val();
if(consultFee>500){
if(confirm('Confirm?')){
alert(123);
}else{
alert(456);
}
}
return false;
});
})
</script>
<asp:button id="btnCalculate" runat="server" text="Calculate Claim" xmlns:asp="#unknown" />