JSFiddle - React, Tailwind, and code Playground

HTML

<div id="CUB">Valor CUB R$ 1.215,40</div>
<div class="calc_cub">
    <input type="number" class="qtd" value="" placeholder="Calcule o CUB" />
    <p>Total R$<span class="total"></span>
    </p>
</div>

JavaScript

$(".calc_cub").on("keyup", ".qtd", function () {
    var valorCub = $("#CUB").text();
    valorCub = parseFloat(valorCub.split('$ ')[1].replace('.', '').replace(',', '.'));
    var qtd = $(this).val();
    $(".total").text(valorCub * qtd);
});