JSFiddle - React, Tailwind, and code Playground

HTML

<select id="custome_vara" name="data[item][custome_vara]" size="1">
    <option value="" id="custome_vara_" selected="selected">alb</option> 
    <option value="20" id="custome_vara_20">stejar auriu</option>
    <option value="30" id="custome_vara_30">moreiche</option>
</select>
<div>
<span class="hikashop_product_price_full" id="hikashop_product_price_full"> 
    <span class="hikashop_product_discount">-10%</span>
    <span class="hikashop_product_price hikashop_product_price_0 hikashop_product_price_with_discount">51,34 €</span> 
    </span>
</div>

JavaScript

function priceNew(val) {
    var discountedPrice = $('.hikashop_product_price_with_discount');
    var percentOff = $('.hikashop_product_discount');
    var price = discountedPrice.text().split(' ');
    var decimalPrice = price[0].replace(',', '.');
    var eyy = parseFloat(decimalPrice) * val / 100 + parseFloat(decimalPrice);
    eyy = eyy.toFixed(2);
    var newPrice = eyy + ' ' + price[1];
    discountedPrice.text(newPrice);
    percentOff.text('-' + val + '%');
    $('.hikashop_product_price').text(newPrice);
}

$('#custome_vara').one('change', function() {
    priceNew($(this + 'option:selected').val());
});