Cost to replace gas filler tube - on 2001 Buick LeSabre

Gas cap no longer seals so EVAP code shows and I need to get the car inspected in NYS.

Asked by for the 2001 Buick LeSabre
A gas cap would cost no more than $15 but is it the gas cap seal or the sealing surface of the filler neck tube? Has it been properly tested to see if their are other leaks in the gas tank, charcoal canister, evap purge solenoid, vent valve ect... A repair shop can test for leaks using a "smoke machine" that generates inert gas and is pumped into the system, if there are leaks you see the smoke excape. If it were the gas cap sealing surface I would put grease on the sealing surface ,repeat the test and see if the smoke leaks out any where. else.
Thanks. I will ask the shop to do this testing and will post back here what they find. I appreciate yor swift response.