It's working for me..try this one

function CheckOtherFees()
{

var str1 = parseFloat(document.getElementById('<%=txtOtherFee.ClientID%>').value);
var str2 = parseFloat(document.getElementById('<%=hdnTotalValue.ClientID%>').value);
var add = str1 + str2;
document.getElementById('<%=txtTotalFee.ClientID%>').value = add.toFixed(2);
}