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

function formatPrice(_number,decimal,separator,decpoint,format_string) 
{ 
    var r=parseFloat(_number)
 
    var exp10=Math.pow(10,decimal); 
    r=Math.round(r*exp10)/exp10; 
    rr=Number(r).toFixed(decimal).toString().split('.');
 
    b=rr[0].replace(/(\d{1,3}(?=(\d{3})+(?:\.\d|\b)))/g,"\$1"+separator);
 
    r=(rr[1]?b+ decpoint +rr[1]:b);
    return format_string.replace('#', r);
}
alert(formatPrice(1000000.65,0,' ',',','# руб.'));
alert(formatPrice(1000000.65,2,' ',',','# руб.'));