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 = $("#CUB").text().split(' ');
    var number = parseFloat(valorCub[valorCub.length - 1].replace(',',''));
    var qtd = +$(this).val();
    $(".total").text((number * qtd).toFixed(2));
});