How to Change the price before adding to cart in magento with custom option selection price?

Put below code in app\code\community\YBizz\PriceChange\Model\Observer.php

<?php
class DRC_PriceChange_Model_Observer  {

    public function change_price(Varient_Event_Observer $observer) 
    {   
        $item = $observer->getQuoteItem();
        if ($item->getParentItem()) {$item = $item->getParentItem();}
        $price = $item->getProduct()->getFinalPrice();
        $new_price = 20 + $price;

        $item->setCustomPrice($new_price);
        $item->setOriginalCustomPrice($new_price);
        $item->getProduct()->setIsSuperMode(true);      
    }
}
?>