JSFiddle - React, Tailwind, and code Playground

HTML

<div class="header">
    	<h1>Time is Money</h1>

    <p>Find out how much time it will take you to buy something!</p>
</div>
<div class="test">$12</div>
<ul>
    <li>$12</li>
    <li>$14.93</li>
    <li>$2</li>
    <li>$dollars</li>
    <li>14.99</li>
    <li>$12,001,121.13111</li>
</ul>

CSS

body {
    font-family: Helvetica, Arial, sans-serif;
}
h1 {
    font-size: 30px;
    color: green;
}
p {
    font-size: 18px;
}

JavaScript

var pageText = document.body.innerHTML;

document.body.innerHTML = pageText.replace(/\$([\d,]+(?:\.\d+)?)/g,
function (string, c1) {
    //If there are commas, get rid of them and record they were there
    var comma = c1.indexOf(',') != -1;
    c1 = c1.replace(/,/g, '');
    //Parse and double
    var value = '' + (parseFloat(c1) * 2);
    //Reinsert commas if they were there before
    if (comma) {
        var split = value.split(".");
        value = split[0].replace(/(\d)(?=(\d{3})+$)/g, "$1,");
        if(split.length > 1)
            value += "."+split[1];
    }
    //Return with dollar sign prepended
    return '$' + value;
});