Oh, I don’t know that it’s that overpriced. You’ll also need to replace the thermostat, as is doesn’t make sense to take your old thermostat out of the old water pump and put it in your new one. List price for a GM water pump is in fact over $300, but a quality aftermarket one can be had for less. Here’s how I’d price it:
Water Pump $216.00
2 Serp belts $80.00
1.5 gallons Dexcool $35.00
2.5 hours labor $225.00
Final Total $658.10
So using an aftermarket pump saves $100. My local Chevy dealer has a labor rate of $109/hour.
Your dealer is in the ballpark, but as always a competent local independent shop should be able to save you a few dollars.