fatklass,
для атрибута data-... в jquery есть метод data();
parseInt не нужен , у вас было умножение ранее, второй цикл избыточен.
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
<script>
$(function() {
var sum = 0;
$(".offer_container").each(function() {
var data = $(this).data()
sum += data.wanted_wood * data.count;
});
alert(sum)
});
</script>
</head>
<body>
<table class="vis" id="own_offers_table">
<tr class="offer_container " id="offer_49051" data-id="49051" data-count="2" data-wanted_wood="150" data-wanted_stone="0" data-wanted_iron="0"></tr>
<tr class="offer_container " id="offer_49052" data-id="49052" data-count="1" data-wanted_wood="0" data-wanted_stone="150" data-wanted_iron="0"></tr>
<tr class="offer_container " id="offer_49053" data-id="49053" data-count="1" data-wanted_wood="150" data-wanted_stone="0" data-wanted_iron="0"></tr>
</table>
</body>
</html>