obviously the service costs will differ every 20k miles. I'm guessing with the new 5 series, the interval is every 20k miles, so you'll probably find it double the price at 40k & 80k than 20k & 40k
My last service (from the dealer who sold it to me) was around €350. It's an old petrol 520. Can't imagine a new one will be any more expensive to service - especially since things rarely go wrong. I've had it nearly 4 years and never a bother (1 puncture).
Having said that, my uncle has a new 5 series and had terrible trouble with the run-flat tyres. He got a puncture while driving in Europe and followed the instructions (50kph etc etc) but it still wrecked the car and had to be towed back to BMW garage and eventually shipped home 3 weeks later.
He's driven 5 series for the last 20yrs and this made him start looking at Lexus/Merc (although he decided Lexus too cramped).