<select id="car">
  <option value="" selected>Выбор авто</option>
  <option value="1" data-price="10000">Авто 1</option>
  <option value="2" data-price="20000">Авто 2</option>
  <option value="3" data-price="30000">Авто 3</option>
  <option value="4" data-price="40000">Авто 4</option>
</select>
<h4></h4>

const output = document.querySelector('h4');
const select = document.querySelector('select#car');
const options = [...select.querySelectorAll('option')];
const onChange = () => output.innerText = options
  .filter(({ value }) => value === select.value)
  .pop()
  .dataset
  .price ?? '';
select.addEventListener('change', onChange);
onChange();