obscurant,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<script src="http://code.jquery.com/jquery-1.9.1.js"></script>
<script>
$(function () {
var sum = 0;
$("#product_table .price").each( function () {
sum += parseInt( $(this).text(), 10 ) * parseInt( $(this).next().text(), 10 )
});
alert(sum);
});
</script>
</script>
</head>
<body>
<table id="product_table">
<tr>
<td class="price">364.00</td>
<td class="quantity">3</td>
</tr>
<tr>
<td class="price">900.00</td>
<td class="quantity">2</td>
</tr>
<tr>
<td class="price">450.00</td>
<td class="quantity">1</td>
</tr>
</table>
</body>
</html>