Do it yourself, change the oil and replace the filters its not rocket science... if you dont know how check youtube it will cost you less than R400(even if you need tools aswell). Take it to a mech for the major this is a minor service
Yeah it'll cost less than R400 because it's done wrong and everything that should have been changed wasn't:
You need 10W40 at least (possible 5W40 for the Astra) = R250
Oil filter + sump sealing washer = ~R55 (for Corsa, Astra might be more)
Air filter = ~R100 (For a Corsa, Astra might be more)
Spark plugs, NGK = ~R120 (For NGK BPR6ES, used in Corsa, Astra will be different plugs but cost should be the same)
Fuel filter = ~R100 (For Corsa fuel filter)
Pollen filter = R?
Additionally the following might need changing depending on the cars age:
Every service - The brakes should be checked, both the disc and pads
Ever 2 years - Brake fluid will needs to be changed
Every 4 years - Engine coolant needs to be changed
Possible for the Astra, definitely for the Corsa:
Every 4 years or 75 000km whichever comes first - Timing belt needs to be changed.
All of these count for the Corsa but everything here except the timing belt is probably the same for the Astra.
Granted all in all the service would cost approx R2000 if every single item on that list is replaced (brake discs and pads are expensive).
GMs quote won't include brakes tho.