можно например так ...
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title></title>
</head>
<body>
<ul>
<li><a>Линк1 (100)</a></li>
<li><a>Линк2 (200)</a></li>
<li><a>Линк3 (300)</a></li>
</ul>
<script language="JavaScript" type="text/javascript">
var a = document.getElementsByTagName('a');
var summ = 0;
for (var i=0; i<a.length; i++) {
summ += +a[i].innerHTML.replace(/.*?\((\d+)\).*?/,"$1");
}
document.write(summ)
</script>
</body>
</html>