Deymos,
<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<input type="number" id="width" value="1.1"/>
<input type="number" id="price" value="500"/>
<button id="len1" value="1">1m</button>
<button id="len2" value="2">2m</button>
<button id="len3" value="2.5">2,5m</button>
<hr/>
<span id="result">0</span>
<script>
var result = document.getElementById('result');
var width = document.getElementById('width');
var price = document.getElementById('price');
document.body.onclick = function (event) {
var target = event.target;
if(!target.id.match(/len[123]/ig)) return;
result.innerHTML = target.value*price.value*width.value;
}
</script>
</body>
</html>