$(document).ready(function() {
    $("#invoiceitems tbody tr").each(function() {
        $(this).change(function() {
            updateTotal();
        });
    });
});

function updateTotal() {

    //Calculate Subtotal for each item
    var quantity = $('.quantity').val();
    var price = $('.price').val();
    var subtotal = parseInt(quantity) * parseFloat(price);
    $('.subtotal').text(subtotal);
    //Calculate Grand Total
    var sum = 0;
    $('.linetotal').each(function() {
        sum += parseFloat($(this).text());
    });
    $('#grandTotal').html(parseFloat(sum));
}​

<table>
<tr>
<td>
<input type="text" name="unitCost[]" id="unitCost_1" value="15.99" class="editable"/>
</td>
<td>
<input type="text" name="quantity[]" id="quantity_1" value="1" class="editable"/>
</td>
</tr>
<tr>
<td>
<input type="text" name="unitCost[]" id="unitCost_2" value="2.50" class="editable"/>
</td>
<td>
<input type="text" name="quantity[]" id="quantity_2" value="1" class="editable"/>
</td>
</tr>
</table>

$(document).ready(function() {
    updateTotal();

    $(".editable").live("change", function() {
        updateTotal();
    });
});