Как вариант...
<!DOCTYPE html>
<html>
<head>
<style type="text/css">
</style>
<script type="text/javascript">
function Calc() {
var t=+document.getElementById('tarif').value
var s=+document.getElementById('srok').value
document.getElementById('summ').value=t*s
}
</script>
</head>
<body>
<select size="1" id="tarif" onchange='Calc()'>
<option value="0"></option>
<option value="10">Тариф 1</option>
<option value="20">Тариф 2</option>
<option value="30">Тариф 3</option>
</select>
<select size="1" id="srok" onchange='Calc()'>
<option value="0"></option>
<option value="1">1 месяц</option>
<option value="2">2 месяца</option>
<option value="3">3 месяца</option>
</select>
<input type='text' id='summ' />
</body>
</html>