BMW have a service schedule, so it will depend on mileage and time since last service (as in - its a service once per year even if barely ridden).
As a guide, mine have been between £165 and £600-ish, which was the large 18k / 36k service.
A normal one seems to be around the £250 mark.
A decent dealer will be able to estimate the expected cost, assuming they don't find anything untoward - and if its already got a full service history, there shouldn't be any nasty suprises really.
What's the mileage, and when was it last serviced?