JSFiddle - React, Tailwind, and code Playground

HTML

<form id='myform' method='POST' action='#'>
    <p>Помада</p>
    <span>Стоимость</span>
    <input type='text' value='300' name="sto" class="sto" disabled >
    <span>Кол-во</span>
    <input type='text' value="1" name="kolvo" class="kolvo">
    <p>цена</p>
    <input type='text' value="" name="cena" class="cena">
</form>

JavaScript

jQuery(document).ready(function(){
  var sto = $(".sto").val();
  var kolvo = $(".kolvo").val();
  $(".cena").val(sto*kolvo);
});