<script type="text/javascript">

var productpr, qty;

function price() {
    var productpr = document.getElementsByName('productpr')[0],
        info = productpr.options[productpr.selectedIndex].value;
    return parseInt( info.split(':')[1], 10 );
}

function quantity() {
    return parseInt(qty.value, 10);
}

function update_subtotal(event) {
    var subtotal = document.getElementById('subtotal').getElementsByTagName('b')[0];
    subtotal.textContent = '$' + ( price() * quantity() ).toFixed(2);
}

window.addEventListener('load', function (event) {
    productpr = document.getElementsByName('productpr')[0];
    qty = document.getElementsByName('qty')[0];
    productpr.addEventListener('change', update_subtotal, false);
    qty.addEventListener('keyup', update_subtotal, false);
    update_subtotal(); // just in-case if they refreshed the page and kept the values
}, false);

</script>