JSFiddle - React, Tailwind, and code Playground

HTML

<div class="productinfo2">
             <h2>test</h2>
             <span class="price">69,9</span>
             <input type="text" class="quantity">    
             <span class="price2">69,9</span>
             <span class="total">Total:</span>
             <div class="line2"></div>
</div>

JavaScript

$('.quantity').keyup(function () {
        var parent = $(this).parent(),
            price = parent.find('.price').html().replace(',', '.'),
            quantity = parent.find('.quantity').val().replace(',','.'),
            result = parseFloat(price) * parseFloat(quantity);
        parent.find('.price2').html(result);
    });