JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://code.jquery.com/jquery-3.1.1.min.js"></script>
<input type="number" id="quantity" name="quantity" placeholder="Кол-во" value="1" min="1">
<div id="summa"></div>

JavaScript

$(document).ready(function() {
    $('#quantity').change(function(){
         calculate();
    });;
    function calculate(){
    	var summa = 0;
        var quantity = parseInt($('#quantity').val()); 
 				switch (quantity) {
          case  quantity < 5:
            summa = quantity*10;
            break;
          case  quantity > 5:
            summa = quantity*2;
            break;
          default: summa = 444;
        }
        $('#summa').text(summa);
    }
})