jQuery help - pricing & discounts

Hi all,

Hope everyone has had a good week.....

My question comes in the form of I'm trying to get one figure the minus another figure. I'm using jQuery for this has I haven't got around to re-learning Vue just yet.

So lets say I have a figure of $27.99 then the customer applies a discount code of $5 that would equal $22.99 right? Based on this (slightly messy hack) where am I going wrong?

$("#success").hide();
$("#error").hide();
var totalPrice = $("#totalPricePHP").text().replace("£", "").replace(".", "");

$("#validateCoupon").click(function(e) {
    e.preventDefault();
    $.ajax({
        type: "POST",
        url: '/validateCoupon/' + $('#coupon').val(),
        data: {
            code: $('#coupon').val()
        },

        success: function(data) {
            console.log("Total: " + totalPrice);
            console.log("Discount: " + data.amount);

            if (data != '') {
                $("#error").hide();
                $("#totalPricePHP").hide();
                $("#success").show().append("Great! £" + data.amount + " has been deducted.");
                $("#success:disabled");
                $("#totalPrice").html("£" + Math.round(data.amount - totalPrice));
            }
        },
        error: function(data){
            console.log(data);
            $("#success").hide();
            $("#error").show().append("Something was wrong with the coupon you entered.");
        }

    });
});

Idea's to improve this messy lot, will also be grateful, but just looking to get it working and formatting in a currency correctly first.