@{
    ViewData["Title"] = "Создание заказа";
}
<div class="text-center">
    <h2 class="display-4">Создание заказа</h2>
</div>
<form method="post">
    <div class="row">
        <div class="col-4">Кондитерское изделие:</div>
        <div class="col-8">
            <select id="pastry" name="pastry" class="form-control" 
                asp-items="@(new SelectList(@ViewBag.Pastries,"Id", 
                "PastryName"))"></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>
    $('#pastry').on('change', function () {
        check();
    });
    $('#count').on('change', function () {
        check();
    });
    function check() {
        var count = $('#count').val();
        var pastry = $('#pastry').val();
        if (count && pastry) {
            $.ajax({
                method: "POST",
                url: "/Home/Calc",
                data: { count: count, pastry: pastry },
                success: function (result) {
                    $("#sum").val(result);
                }
            });
        };
    }
</script>