JSFiddle - React, Tailwind, and code Playground

HTML

<form action="#">
  <span id="price">2 490 грн</span>
  <fieldset>
    <label for="doorsSize">Суммировать к цене</label>
    <select id="doorsSize">
      <option value="0">0</option>
      <option value="10">10</option>
      <option value="20">20</option>
    </select>
  </fieldset>
   <fieldset>
    <label for="doorsSize">Суммировать еще к цене</label>
    <select id="doorsSize">
      <option value="0">0</option>
      <option value="30">30</option>
      <option value="40">40</option>
    </select>
  </fieldset>
</form>

JavaScript

var itemPrice = document.getElementById('price').innerHTML;
console.log(itemPrice);