, , live. live() DOM, . , , , DOM "", "", , .
Try:
$('.percentage').live('change keyup focus click keydown', function() {
var sum = 0;
$(".percentage").each(function() {
sum += $(this).val();
});
alert(sum);
});
, val() , :
$(".percentage").each(function() {
sum += parseInt($(this).val(), 10);
});