http://learn.javascript.ru/play/xYLh9b
<!DOCTYPE HTML>
<html>
<head>
<style type="text/css">
#result label {
clear: both;
display: block;
margin: 10px 0;
width: 200px;
}
#result input {
float: right;
}
</style>
</head>
<body>
<div id="result">
<label>120 руб
<input data-price="120" type="checkbox">
</label>
<label>123 руб
<input data-price="123" type="checkbox">
</label>
<label>23 руб
<input data-price="23" type="checkbox">
</label>
<div>Итого: <span id="result-sum">0</span></div>
</div>
<script type="text/javascript">
(function() {
var i, total = 0,
resultElem = document.getElementById('result'),
resultLabels = resultElem.querySelectorAll('input[type="checkbox"]'),
resultSum = document.getElementById('result-sum');
function getSum() {
var price = +this.getAttribute('data-price');
total += this.checked ? price : -price;
resultSum.innerHTML = total;
}
for(i=0; i<resultLabels.length; i++) resultLabels[i].onchange = getSum;
})();
</script>
</body>
</html>