What is the EGR control switch? on 1996 Geo Prizm

I looked up quotes for repair on the EGR Modulator. This site says the high end is $600 including labor, but the Midas shop (LA) that has my car says $800 and said I needed a Control switch part, and another Midas shop says high $600s but didn't have anything on their computer about a control switch. Was the first shop pulling my leg?
I have had to have work done to pass smog and I wonder if this problem was the fix all along. I think Pinging is occurring now, and any work over 450 necessitates a new car.

Asked by for the 1996 Geo Prizm
The EGR on you GEO (similar to a Toyota Corolla has several parts to the EGR system. It's got a valve, a few hoses , a vacuum modulator valve, and a vacuum control solenoid. There are tests to be done on each to determine which part of the system has failed. Applying vacuum to the EGR port with the engine running should stall the engine if the valve works, the solenoid is easy to test by applying power and ground and then testing for power or ground to see if the computer is working. If the parts are hard to source from Geo write down the Nippon- Denso part number on the part itself and see if it can be cross referenced to a Toyota number.