@{
    ViewData["Title"] = "Привязка статьи затрат к сделке";

    var costs = new SelectList(ViewBag.Costs, "Id", "NameOfCost");
    costs.Where(x => Convert.ToInt32(x.Value) == ViewBag.SelectedId).ToList().ForEach(x => x.Selected = true);
}
<div>
    <h2 class="display-4">@ViewData["Title"]</h2>
</div>
<form method="post">
    <div class="row mb-2">
        <div class="col-2">Статья затрат:</div>
        <div class="col-sm-10">
            <select id="cost" name="cost" class="form-control" asp-items="@costs"></select>
        </div>
    </div>
    <div class="row mb-2">
        <div class="col-2">Сделки:</div>
        <div class="col-sm-10">
            <select id="purchase" name="purchase" class="form-control" asp-items="@(new SelectList(@ViewBag.Purchases, "Id", "Id"))"></select>
        </div>
    </div>
    <div class="row mb-2">
        <div class="col-2">Количество:</div>
        <div class="col-sm-10">
            <input type="text" class="form-control" name ="count" id="count" placeholder="Количество">
        </div>
    </div>
    <div class="row mb-2">
        <div class="col-2">Итоговая сумма:</div>
        <div class="col-sm-10">
            <input type="text" class="form-control" id="resultSum" placeholder="Итоговая сумма" readonly>
        </div>
    </div>
    <div class="text-center mb-2">
        <input type="submit" value="Привязать" class="btn btn-primary" id="submit" />
    </div>
</form>

@section Scripts
    {
        <script>
        $('#cost').on('change', check);
        $('#count').on('change', check);

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