ЯННП что значит "суммировать квадратуру", но смысл такой:
<html>
<head>
<meta charset="windows-1251" />
</head>
<body >
<span id="itogo">50 <span>кв\м</span></span>
<span id="itogo">23 <span>кв\м</span></span>
<span id="itogo">56 <span>кв\м</span></span>
<span id="itogo">1233 <span>кв\м</span></span>
<span id="itogo">234.2 <span>кв\м</span></span>
<span id="itogo">676.15 <span>кв\м</span></span>
<span id="itogo">76.26 <span>кв\м</span></span>
<span id="itogo">23.8 <span>кв\м</span></span>
<span>сумма:<span class="summavse">0</span></span>
<script>
with(document){
querySelector(".summavse").innerHTML=[]
.map.call(querySelectorAll("#itogo"), function(x){
return parseFloat(x.innerHTML)
})
.reduce(function(x, y){return x+y})
}
</script>
</body>
</html>