@{
	ViewData["Title"] = "Create";
}
<div class="text-center">
	<h2 class="display-4">Создание заказа</h2>
</div>
<form method="post">
	<div class="row">
		<div class="col-4">Платье:</div>
		<div class="col-8">
			<select id="dress" name="dress" class="form-control" asp-items="@(new SelectList(@ViewBag.Dresses,"Id", "DressName"))"></select>
		</div>
	</div>
	<div class="row">
		<div class="col-4">Количество:</div>
		<div class="col-8"><input type="text" name="count" id="count" /></div>
	</div>
	<div class="row">
		<div class="col-4">Сумма:</div>
		<div class="col-8"><input type="text" id="sum" name="sum" readonly /></div>
	</div>
	<div class="row">
		<div class="col-8"></div>
		<div class="col-4"><input type="submit" value="Создать" class="btn btn-primary" /></div>
	</div>
</form>
<script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
<script>
	$('#dress').on('change', function () {
		check();
	});
	$('#count').on('change', function () {
		check();
	});

	function check() {
		var count = $('#count').val();
		var dress = $('#dress').val();
		if (count && dress) {
			$.ajax({
				method: "POST",
				url: "/Home/Calc",
				data: { count: count, dress: dress  },
				success: function (result) {
					$("#sum").val(result);
				}
			});
		};
	}
</script>