оч просто
<html>
<head>
$('#start').live('click', function(){
var total = 0;
$('table .weight').each(function(){
total += Number($(this).text());
});
$('table .total').html( total );
});
</script>
</head>
<body>
<table id="tovar">
<tbody>
<tr>
<td>товар1</td><td class="weight">20</td>
</tr>
<tr>
<td>товар2</td><td class="weight">40</td>
</tr>
<tr>
<td>товар3</td><td class="weight">60</td>
</tr>
</tbody>
<tfoot>
<tr>
<th colspan="2" class="total"></th>
</tr>
</tfoot>
</table>
<input type="button" id="start" value="посчитать вес">
</body>
</html>