Как вариант...
<!DOCTYPE html>
<html>
<head>
<meta http-equiv='Content-Type' content='text/html; charset=windows-1251' />
<script src='http://code.jquery.com/jquery-latest.js'></script>
<!--
<script src="https://code.angularjs.org/1.3.9/angular.min.js"></script>
<script src="https://code.angularjs.org/1.3.9/angular-route.js"></script>
<link rel='stylesheet type=text/css href=tmp.css' />
<link rel="stylesheet/less" type="text/css" href="style.less">
<script src="http://cdnjs.cloudflare.com/ajax/libs/less.js/2.5.0/less.min.js"></script>
-->
<style type='text/css'>
</style>
<script type='text/javascript'>
$(function(){
var totalSum = 1000;
$('#sborka, #dostavka').click(function(event) {
var n=parseInt(this.value);
n=(this.checked)? n: -n;
totalSum += n;
$('#totalsum').html(totalSum);
});
$('#totalsum').html(totalSum);
});
</script>
</head>
<body>
<input type="checkbox" value="500" name="sborka" id="sborka">
<input type="checkbox" value="300" name="dostavka" id="dostavka">
<p id='totalsum'></p>
</body>
</html>