qumo,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
<script>
$(function() {
var res = document.getElementById("answer"),
price = {
"ems": 2200,
"avia": 2500,
"sal": 2700,
"parohod": 1800
};
res.innerHTML = price["ems"];
function question() {
var method = $(".find").val();
var weight = +$("#weight").val()||0;
var sum = price[method];
switch (method) {
case "ems":
for (var i = 50; i < weight * 100; i++) sum += i < 100 ? 20 : 18;
break;
case "avia":
break;
case "sal":
break;
case "parohod":
break
};
res.innerHTML = sum
}
$("#weight").on("change", question);
$(".find").on("change", question)
});
</script>
</head>
<body>
<select name="method" class="find" >
<option value="ems">ЕМS</option>
<option value="avia">Авиа</option>
<option value="sal">SAL</option>
<option value="parohod">Пароход</option>
</select>
<input type="number" value="0.5" min="0.5" step="0.01" max="2" name="weight" id="weight" size="4" >
<hr><span id="answer"></span> руб
</body>
</html>