Indeed even a quality oil and filter shouldnt cost more than £50 if you change it yourself.
Plenty of branded gear around too. I'd take a look at Opieoils.com to give you a wide range of oils and pretty decent prices. Espcially when they are doing their deals.
If my car was turbo'd I'd lower the service intervals as turbo's do give oil a hard time.
Reading through the 320d manual my dad recently purchased (Great car by the way. feels very much like the alfa but with a much more drab interior and exterior even in M Sport) I was shocked to see the Gearbox / Diff oil never needs changing which is interesting to say the least.
Alfa 166 2.0 Twinnie - LPG and a few trick performance parts