JSFiddle - React, Tailwind, and code Playground

HTML

<div id='calculate' class="calc">
	<select id='price'>
		<option value = "100">100р.</option>
		<option value = "200">200р.</option>
		<option value = "300">300р.</option>
		<option value = "400">400р.</option>
		<option value = "500">500р.</option>
	</select>
	<select id='count'>
		<option value = "1">1шт.</option>
		<option value = "2">2шт.</option>
		<option value = "3">3шт.</option>
		<option value = "4">4шт.</option>
		<option value = "5">5шт.</option>
	</select>
	<div id="label">Стоимость</div><div id="result"></div>
</div>

CSS

html, body{margin: 0px; background-color: #20262e;}
.calc{
	width: 220px;
	height: 250px;
	margin: 0px  auto  0px  auto;
	background-color: #989898;
	color: #515353;
	padding: 10px;
	font-size: 20px;
}

#label{
	margin: 20px  0px  -15px  0px;
}

select, option, #product,#count,#result{
	width: 100%;
	height: 40px;
	margin: 20px 0px 0px 0px;
	background-color: #ffe9e9;
	border: 1px;
	border-radius: 10px;
	outline:  none;
	padding: 10px;
}

#result{
	width: 90%;
	height: 30px;
	padding: 10px 10px 2px 10px;
}

JavaScript

calculate.onclick = function() {
  const price = document.getElementById('price').value;
  const count = document.getElementById('count').value;
  document.getElementById('result').innerHTML = price * count;
}