JSFiddle - React, Tailwind, and code Playground

HTML

<div class="content_half float_r">
    <form id="add2cart" name="cart" method="Post" action="<?php fetchdir($bpages); ?>cart.php">
        <table>
            <tr>
                <td width="160">Price:</td>
                <td name="price" id="price">£<span id="priceDisplay"><?php echo $row [ 'Start_Cost']; ?></span>

                </td>
            </tr>
            <tr>
                <td for="size" width="160">size*:</td>
                <td>
                    <select name="size" class="small" id="size"></select>
                </td>
            </tr>
            <tr>
                <td>size: <span id="sizeDisplay"></span>

                </td>
            </tr>
            <tr>
                <td>Quantity</td>
                <td>
                    <input type="text" id="qty" name="Qty" value="1" style="width: 20px; text-align: right" />
                </td>
            </tr>
            <tr>
                <td>Total</td>
                <td>£<span id="totalDisplay"></span>

                </td>
            </tr>
        </table>
        <div class="cleaner h20"></div>
        <input type="hidden" name="pid" id="id" value="<?php echo $id; ?>" />
        <input type="Submit" class="button" name="button" id="button" value="Add to shopping cart" />
    </form>
</div>

JavaScript

var shoeData = {
    12: 25,
    14: 30,
    16: 35
}

$(function () {

    $(document).ready(function () {
        $.each(shoeData, function (key, value) {
            $("#size").append($("<option>", {
                "value": key,
                    "text": key + " size(£" + value + ")"
            }));
        });
        updateHandler();
    });


    $("#size").on('change', updateHandler);
    $("#qty").on('input', updateHandler);
});

function updateHandler(e) {
    // accumulate the data
    var value = $("#size").val();
    var quantity = $("#qty").val();
    var price = shoeData[value];
    var endPrice = price * quantity;

    // display the data
    $('#priceDisplay').html(price);
    $("#sizeDisplay").html(value);
    $("#totalDisplay").html(endPrice);
}