I think I paid right around $375 for my kit and it took about 90 minutes to install because the exhaust has to be tapped and a bung welded in for the new sensor to go in. Out of curiosity, why are you considering an A/F ratio meter?
"Posterity! You will never know how much it cost the present Generation to preserve your Freedom! I hope you will make good use of it. If you do not, I shall repent in Heaven, that I ever took half the Pains to preserve it."