How much are these for a 97 bonnie? And when should they be replaced. I juts hit 50k miles.

Only change #1 (in the exhaust manifold) not #2 (behind the cat). #1 can affect performance and effiiciency. Most seem to be degrading at about 50k-60k. The part number is listed in Techinfo. Just run that through www.gmpartsdirect.com , as that will be the best price you can get on an AC Delco O2. I don't suggest an aftermarket brand.