Preis im Warenkorb neu berechnen

Zuerst in config.xml an Event hängen

[..]
	<events>
		<checkout_cart_product_add_after>
		    <observers>
		        <giftwrap_checkout_cart_product_add_after>
		            <class>giftwrap/observer</class>
		            <method>checkout_cart_product_add_after</method>
		        </giftwrap_checkout_cart_product_add_after>
		    </observers>
		</checkout_cart_product_add_after>
	</events>
[..]

Danach im Observer-Model eigenen Preis setzen

<?php
class Webguys_Giftwrap_Model_Observer {

	public function checkout_cart_product_add_after( $event ) {
             $quote_item = $event->getQuoteItem();
             /* @var $quote_item Mage_Sales_Model_Quote_Item */
             $price = 1234;
             $quote_item->setCustomPrice( $price );
             $quote_item->setOriginalCustomPrice( $price );
	}

}
?>