zoomx25,
<!DOCTYPE html>
<html>
<head><meta charset="utf-8">
<style type="text/css">
</style>
<script type="text/javascript">
function calc_comlect() {
var days3= +document.getElementById('days3').value; // кол-во дней
var gsm = document.getElementById('selectId').value == "rtk_gsm";
var sumDay = gsm ? [5500,4500,4000,3500] : [6000,5000,4500,4000];
var day = [3,7,14,30];
sumDay = day.reduce(function(a,b,i) {
return b <= days3 ? sumDay[i] : a
},0)
document.getElementById('summ').value=sumDay *days3 // сумма дни + комплект
}
</script>
</head>
<body>
<select size="1" id="selectId" onchange="calc_comlect()">
<option value="rtk_uhf">UHF 35W</option>
<option value="rtk_gsm">GSM/GPRS</option>
</select>
<input type="text" name="days3" id="days3" /> <!-- ввод кол-во дней -->
<input type='text' id='summ' /> <!-- вывод суммы на экран -->
<input type="button" name="button" id="button" onClick="calc_comlect()" value="вычислить">
</body>
</html>