An OEM lug nut is about $5 - and an OEM wheel stud is probably about the same. The $80 total charge probably includes labor to remove and install the stud. Some wheel studs are very easy to remove and install - some require more time.
I would ask the repair shop why the stud and two lug nuts needed to be replaced. Was the stud cross threaded? If so, that would also damage the one lug nut. What was wrong with the second lug nut?
I suspect the person that installed the spare tire cross threaded the lug nut onto the wheel stud. If it had already been damaged, the person should have pointed it out to you (so they didn’t get blamed for the damage).
Some mechanics do not hand thread the lug nut onto the wheel stud - they place the nut into the socket and quickly spin it on the wheel stud. If it isn’t started correctly, the wheel stud threads will be damaged. That is possibly what happened.
Let us know what the shop tells you.
PS - I own a 2014 Impala 2LTZ, and previously owned a 2008, 2011 and 2012 Impala LTZ; all purchased new.