Do not use a main dealer is a good guide ! Always use a local independent garage and ask friends for recommendations.
As long as there are 'stamps' in the service book, the car will hold it's value.
Regular servicing should not cost a lot unless there are repairs done.
If the car is older, consider just getting the oil, oil filter and air filter changed, these are the 'life blood' of any engine and don't cost much at all.
Most reputable garages display their charges for servicing and other routine work such as brakes, tyres, exhausts etc. so you should know in advance what you'll pay.