I had the 20,000 mile service done in June at no cost to me.....although it was just an oil change, fluids service and basic checks (plus some needed updates). When it comes time to do the 30,000 mile service I will go to my local chain I've been going to for 25 years. They service a few local Giulias that also bypass the dealership.....becuase they are fast, cheap and do great work. So, I'd expect to pay half or less what the Alfa dealer would charge for the spark plug service.
I will hit 50,000 miles in just over a year. I haven't even looked into what that service will entail......