If anything it is better than the 2.5 v6 (less effort needed to get it up to a decent turn of speed)
Expect at worst 18mpg if you flog the living hell out of it
22mpg in town driving and a surprisingly pleasant 32 mpg on a steady 70-80mph motorway cruise.....
But... lets be honest, you don't ever buy a V6 and worry about fuel economy! That's what they aren't about... Be cheap and buy a JTD if that's a concern
Brakes are pretty reasonable despite the Brembo tag and the pads are even easier for a DIY'er to change than TS Alfa's!
Exhausts again aren't much more of a cost and seem to last longer...
Where the V6 does become costly is the servicing..... Spark plugs last a long time but the sheer task of changing the rear bank of three, replacing the impossibly difficult to get at oil filter and the dreaded cam belt, tensioner water pump and guide rollers are the cost killers albeit longer periods than a TS at a safe 60,000 miles.....
Then there is the clutch plate. Hmmm
80,000 miles if you are very lucky indeed but a nightmare to replace.
Look on the bright side though.
You own a fantastic car with an even more fantastic engine that is stronger and far more reliable than a twin spark or a JTD as long as its looked after properly
Here is a link with some ball park figures for you to digest:
Alfa Romeo Servicing