Result

by Result

HTML

<p>Количество:</p>

<div class="number">
  <input id="count" type="text" value="1">
  <button id="plus">+</button>
  <button id="minus">-</button>
</div>

<p>Цена:</p>
<div><span id="price">2 500</span>руб.</div>

JavaScript

$('#plus').on('click', function() {

  count = parseInt( $('#count').val() ) + 1;

  $('#count').val(count);
  updatePrice(count);
});

$('#minus').on('click', function() {

  count = parseInt( $('#count').val() ) - 1;
  count = count < 1 ? 1 : count;
  
  $('#count').val(count);
  updatePrice(count);
});

function updatePrice(count) {

  $('#price').text(2500 * count);
}