JSFiddle - React, Tailwind, and code Playground

by Thiago Diniz

HTML

<span class="shelf-best-price" itemprop="price">R$ 498,90</span>
<p class="flag desconto-a-vista-10-">Desconto à vista 5%</p>

<div class="novo-preco"></div>

JavaScript

var price = $('.shelf-best-price').text().replace(/[\D]+/g, '');
var priceFloat = parseFloat(price);
var discount = $('.flag[class*="desconto-a-vista"]').text().replace(/[^0-9]/g, '');
var subTotal = priceFloat * discount / 100;
var total = price - subTotal;

total = (total + '').replace(/([0-9]{2})$/g, ",$1");

if (total.length > 6) {
  total = total.replace(/([0-9]{3}),([0-9]{2}$)/g, ".$1,$2");
}

$('.novo-preco').text('R$ ' + total);