Check the collectRates() method in Mage_Usa_Model_Shipping_Carrier_Ups inside this method you will find $this->_result = $this->_getQuotes();
Further debugging tells us $this->_result is of type Mage::getModel('shipping/rate_result') which is a container for Mage::getModel('shipping/rate_result_method') in which you shoud change the price with setData('price') or maybe setData('cost');
cheers!