What happens when those codes get thrown is that the PCM uses the one known good one for fuel trim (goes into a semi-open loop mode), and errs on the side of caution, using more gas to ensure the motor doesn't run lean. Hence the lower mileage. You can't fake out the upstream sensor.
Replacing that one isn't so bad, its the one on the radiator side of the engine, closest to the engine. Picking up an O2 socket from an auto parts store will be handy (it has a slot in it to allow the wires to stick out of the socket during removal/install). J-Lab is right about the price, and recommending replacement. Just wait until it is cold, since the manifold and the sensor will be hot (sensor is heated to over 600 degC).
