Observer.php

    /**
     * Set prices and image configuration
     *
     * @param Varien_Event_Observer $observer
     */
    public function salesQuoteCollectTotalsBefore(Varien_Event_Observer $observer)
    {
        /* @var $quote Mage_Sales_Model_Quote */
        $quote = $observer->getQuote();
        foreach ($quote->getAllItems() as $quoteItem) {
            /* @var $quoteItem Mage_Sales_Model_Quote_Item */
            $product = $quoteItem->getProduct();
            $price = $helper->getPriceWithoutTax($quoteItem);
            $product->setPrice($price);
        }
    }