You don't buy Alfas for their good fuel economy (unless you buy a JTD), but even 500 km range is pretty low.
500 km is only 'normal' when you drive around town most of the time. Within the city limits the modern day Alfa is a real gas guzzler. If you drive a lot outside of town in a 147 1.6 TS Veloce, you should be able to reach an average of 10l / 100 kms with a reasonabily sporty driving style.
BTW Don't forget to visit
www.selespeed.info as a 147 owner.