JSFiddle - React, Tailwind, and code Playground

HTML

<input id="i1" class="q-mini" type="number" min="1" max="99" onchange="f()" name="quantity" size="2" value="1" data-bme="Количество"/> 

<input id="i2" oninput="f()" type="hidden" value="44.33"  name="pk_price" data-bme="Цена"><span class="price-new">44.33</span> 

    <input id="i3" value="44.33">

JavaScript

function f() { 
    var n1=document.getElementById('i1').value; 
    var n2=document.getElementById('i2').value; 
    document.getElementById('i3').value=(1*n1*n2).toFixed(2); 
}