JSFiddle - React, Tailwind, and code Playground

HTML

<span id="totalprice">$10</span>

<select id="upgrade1">
    <option value="10">A</option>
    <option value="20">B</option>
</select>
<select id="upgrade2">
    <option value="10">C</option>
    <option value="20">D</option>
</select>

JavaScript

$('#upgrade1, #upgrade2').on('change', function () {
    sum = parseInt($('#upgrade1').val()) + parseInt($('#upgrade2').val());
    $('#totalprice').html('$' + sum);
});