I think we could lessen the amount of work we have to do if we replace the HTML comment with a hidden input like this:
<input type="hidden" class="additional_price" value="20"/>
Then we can add it to the span with this JavaScript:
$(document).ready(function(){
var priceElement = $('span.cena');
var originalPrice = parseInt($(priceElement).text().replace(/,/g, ''));
var additionalPrice = parseInt($('.additional_price').val());
var totalPrice = originalPrice + additionalPrice;
if (totalPrice < 0) {
totalPrice = 0;
}
if (!isNaN(totalPrice)) {
$(priceElement).text(' ' + totalPrice + '.99');
}
})