PonyS,
Совсем не понял смысла этих расчетов, но как-то так...
<!DOCTYPE html>
<html>
<head>
<title>Untitled Page</title>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.0/jquery.min.js"></script>
<script>
$(function () {
var sales = 0;
$(".sdg").change(function () {
sales = this.value;
calc();
});
var calc = function () {
var sum = 0;
$(".s2,.text2,.sum[type='text'],.sum:checked", fields).each(function () {
sum += +this.value || 0
});
var sale = 0;
$(".sale:checked", fields).each(function () {
sale += +this.value
});
$("#text").val(sales * (sum - sum * sale / 100).toFixed(2))
};
fields = $("form");
fields.on('input change', calc);
});
</script>
</head>
<body>
<form action="" method="POST" id="form" style="">
<select name="s2" class="s2">
<option value="0" selected="selected">How many?</option>
<option value="90">90$</option>
<option value="95">95$</option>
<option value="120">120$</option>
<option value="150">150$</option>
<option value="180">180$</option>
</select>
<select name="text2" class="text2">
<option value="0" selected="selected">How many?</option>
<option value="0">0$</option>
<option value="25">25$</option>
<option value="50">50$</option>
<option value="75">75$</option>
<option value="100">100$</option>
</select>
<select name="text22" class="text2">
<option value="0" selected="selected">Number</option>
<option value="1">1m</option>
<option value="2">2m</option>
<option value="3">3m</option>
<option value="4">4m</option>
<option value="5">5m</option>
<option value="6">6m</option>
</select>
<select name="sdg" class="sdg">
<option value="0" selected="selected">Duration</option>
<option value="3">3h</option>
<option value="4">4h</option>
<option value="5">5h</option>
<option value="6">6h</option>
<option value="7">7h</option>
</select>
Расчет -
<input type="text" value="0.00" id="text">
</form>
</body>
</html>