JSFiddle - React, Tailwind, and code Playground

HTML

<form id="quantityform" method="post" action="#">
    <input type="hidden" name="setcost" value="50" />
    <label for="currentcost">Total cost: </label><input type="text" id="currentcost" name="currentcost" value="" />
    <label for="quantity">Quantity: </label><input type="text" value="" id="quantity" name="quantity" placeholder="How many items would you like?" />
    <input type="submit" value="Place order" />
</form>

JavaScript

$(function() {
    $("[name=quantity]").blur(function() {
            var setval = $("[name=setcost]").val();
            var quantity = $("[name=quantity]").val();
           var total = setval*quantity;
        $("[name=currentcost]").val(total);
    });
})