how much does it cost to replace freeze plugs

It's all "flat rate" labor - there is no repair procedure published nor is there a labor time. It could be an hour or so, could be several hours depending on the location and how much corrosion is involved. Have a local shop assess it and give you a first hand quote. Good luck. Some shops near you: https://repairpal.com/chevrolet-repair-in-absecon-new-jersey