JSFiddle - React, Tailwind, and code Playground

HTML

<select class="form-select" id="foo" data-price="5000">
<option value="#" selected="selected" disabled="">Выбрать</option>
<option value="0">Всадной</option>
<option value="1400">Сквозной (+1400)</option>
<option value="1000">Цельнометаллический (+1000)</option>
</select>

<pre id="display"></pre>

CSS

pre {
  margin: 1em;
  padding: 1em;
  outline: dashed 1px gray;
}

JavaScript

foo.addEventListener('change', function() {
  const startPrice = parseInt(this.dataset.price, 10)
  const value = parseInt(this.value, 10)
  display.textContent = startPrice + value
})