JSFiddle - React, Tailwind, and code Playground

by Andrey7287

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;

var toNum = parseInt(itemPrice.split(" ").join(""));
console.log(toNum);
var toStr = toNum.toString().split("");
if (toStr.length > 3) {
 toStr.splice(-3, 0, " ");
}
toStr = toStr.join("") + " грн";
console.log(toStr);