What were the numbers on your emissions test and the limits?
Did you have high HC? or something else?
As mentioned, O2 sensors are not $300. You may want to try my friend's garage: GRAF AUTO, 142 Bentworth Ave. 416-789-3423, Ask for Rudy Graf. He is very reasonable. Also, he will soon be able to do dyno runs on his emissions dyno, as soon as I help him calibrate it.