Try:
<script>
function updatePrice() {
var selVal = document.getElementById('grossSelect').options[document.getElementById('grossSelect').selectedIndex].value;
if (selVal == '0') {
document.getElementById('enterGross').style.display='block';
baseVal = 660;
enteredAmount = document.getElementById('gross').value;
enteredAmount = enteredAmount.replace(/[^\d]/g,"");
enteredAmount = parseInt(enteredAmount);
if (enteredAmount > 500000) {
enteredAmount = enteredAmount - 500000;
baseVal += 35 * (enteredAmount/100000);
}
document.getElementById('calcTot').innerHTML= baseVal.toFixed(2);
} else {
document.getElementById('enterGross').style.display='none';
document.getElementById('calcTot').innerHTML=selVal.toFixed(2);
}
}
</script>