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"/><input type="number" id="price" value="333"/>
<button name="len" value="1">1m</button>
<button name="len" value="2">2m</button>
<button name="len" value="2.5">2,5m</button>
<hr/>
<span id="result">0</span>
<script>
!function() {
var result = document.getElementById('result'),
width = document.getElementById('width'),
price = document.querySelectorAll('#price')[1];
document.body.onclick = function(event) {
var target = event.target;
if(target.name == 'len') {
result.innerHTML = target.value * price.value * width.value;
}
}
}();
</script>
</body>
</html>