[Rome.]

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.11.2/jquery.min.js"></script></script>
<style>
  .wrap-count  {
    margin: 100px;
  }
  .wrap-count > input {
    display: inline-block;
  }
  input[type=text] {
    width: 30px;
  }
  #change_val {
    width: 100px;
  }
</style>

  <div class="wrap-count">
   <input type="button" id="minus_count" value="-">
   <input type="text" id="field_count" value="36">
   <input type="button" id="plus_count" value="+">
   <input type="text" id="change_val" value="1390 руб.">
  </div>
  <script>
    var valInput = $('#field_count').attr('value');
    valInput = Number(valInput);
    var changeVal = $('#change_val').attr('value');
    changeVal = changeVal.replace(/\D/g, '');
    changeVal = Number(changeVal);
    console.log(changeVal)
    var step = 2;
    var price = 50;

    $('#plus_count').click(function () {
     valInput = valInput + step;
     changeVal = changeVal + price;
     $('#field_count').attr('value', valInput);
     $('#change_val').attr('value', changeVal + ' руб.');
      if (valInput > 160) {
         valInput = 160;
         $('#field_count').attr('value', 160);
      }
      if (changeVal > 4490) {
         changeVal = 4490;
         $('#change_val').attr('value', changeVal + ' руб.');
      }
    });

    $('#minus_count').click(function () {
     valInput = valInput - step;
     changeVal = changeVal - price;
      $('#field_count').attr('value', valInput);
      $('#change_val').attr('value', changeVal + ' руб.');
      if (valInput < 36) {
         valInput = 36;
         $('#field_count').attr('value', 36);
      }
      if (changeVal < 1390) {
         changeVal = 1390;
         $('#change_val').attr('value', changeVal + ' руб.');
      }
    });
  </script>