Yeah, it is apparently a big job on the 3rd service. But, it's a £60k+ car so what's the issue?
Alfa might be jumping the gun with an early belt change but it's better that than lots of early failures.
It is only an accessory belt not a timing belt as the V6 engine has a chain, so it does strike me as expensive.
Also there may be a difference between US and UK service intervals.
According to my owner's manual the service interval for the Quadrifoglio is 9,000 miles, (strictly 15,000 km) and it is the fourth service, ie 36,000 miles, at which the belt must be changed with a caveat that it might need changing earlier:
"Areas that are not dusty: advised maximum distance 36,000 mi (60,000 km). The belt must be replaced every 4 years, regardless of distance travelled.
Dusty areas and/or heavy conditions (cold climates, urban driving, long periods of idling): the recommended maximum distance is 18,000 mi (30,000 km). The belt must be replaced every 2 years, regardless of distance travelled."
My belief is that it's the spark plug change which drives the fourth service cost.
Exactly as ecain63 says if you can afford to buy a Quadrifoglio the service cost shouldn't be something to worry about.
In my 13 years and 176,000 miles of 156 GTA ownership, petrol, repairs and servicing must have amounted to more than the purchase price.