JSFiddle - React, Tailwind, and code Playground

HTML

<p>Item Amount: Php 1499</p>
<p>Reward Amount: <input type="number" name="rewards-amoung" id="reward-amount" value="0"></p>
    <p>Final Price: Php <span id="demo">1499</span></p>

JavaScript

document.getElementById("reward-amount").onchange = function () {
    var price = 1499 - document.getElementById("reward-amount").value ;
    document.getElementById("demo").innerHTML = price;
}