Chevrolet Repair: Replace Exhaust 2003 Chevy Cavalier

Question
Just bought 2003 Cavalier. In getting brakes done, mechanic said the rattling comes from exhaust system from the resonator to the exhaust pipe. Cost to repair estimated at $900. Ouch!!  Code indicates a faulty downstream O2 sensor that I planned to replace myself.  Mechanic said a waste of time to put in new sensor when the exhaust will eventually fail.
Options available?  Fair price for the work (Chicago)?

Thanks..

Answer
What! $900.00. You should check out some muffler shops. The o2 sensor is not that expensive, so get it. Use it in existing system. It needs it for engine to reduce unburned hydrocarbons(gas)out tailpipe. The catalytic converter will get damaged ,and then you are in the market for a new one of those, and they run around $250-$350.You could have a whole exhaust system hand fabricated for probably $275 and use your old cat and its good to go. And since they don't furnish o2 sensors,with the exhaust pipe.