orb,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
</head>
<body>
<select class="sel" name="" size="1">
<option value="1">1</option>
<option value="50">2</option>
<option value="20">3</option>
</select>
<span class="price" data-num="100"></span>
<span class="price" data-num="200"></span>
<span class="price" data-num="300"></span>
<script>
var sel = document.querySelector('.sel'), price = document.querySelectorAll('.price'),
fn = function() {
var val = this.value;
[].forEach.call(price, function(item) {
item.innerHTML = item.dataset.num * val + "$"
} )};
sel.addEventListener('change', fn);
fn.call(sel)
</script>
</body>
</html>