Oxygen Sensors and EGR Valve (Pricing for parts and labor)

First find out exactly what fault code numbers are set. Diagnostic fault codes never say to replace parts or that they're defective. They only indicate the circuit or system that needs further diagnosis. In the case of oxygen sensors there are dozens of potential codes related to them. Most have to do with a defective condition that is being reported by a properly working sensor. You don't replace the sensor because you don't like what it is reporting. A new one will do the same thing and waste your money.

The same is true of the EGR valve. It could be defective but you need to know the exact fault code number to know if there's an electrical problem, a flow problem, or something else.

Be aware that GM has a lot of tricks built in to their vehicles, especially newer ones, to make a lot of money off of you in repairs that only the dealers can do. Any independent shop can still work on a 2000 model, and high-quality aftermarket replacement parts will cost a lot less than those from the dealer. If you have a mechanic you trust to diagnose the cause of the problem, why are you going to the dealer to have the repairs done?

You also can't go by the estimate you were given because you didn't give them the chance to inspect the car themselves. You didn't ask the dealer for the cost to repair your car, (which they have no way of knowing). You asked them what they would charge to install two parts. No one is going to be happy when those new parts don't solve the problem but you will be obligated to pay for the services you asked for. The dealer will not be responsible for the mis-diagnosis because you didn't allow them the chance to diagnose it. They just did what they were told to do.

To further complicate things, if you had your mechanic read the fault codes soon after the Check Engine light turned on, and they refer to two totally different parts, what are the chances both failed at exactly the same time? It's much more likely there's a problem with the EGR system resulting in a problem with the exhaust system that's being reported by the oxygen sensor. That would support my concern that a new oxygen sensor is not needed. Lets start with the exact fault code numbers.