300 out of 475 is for workmanship, becos they have to open up ur dashboard before can take out the condenser to service.
A good workshop will tell you the truth that is not worth to service aircond, the shop you went most probably want to con your money.

Don't fix it if not broken. Our aircond no need service one. if you feel not cold, you can ask to check for coolant level, they will estimate if there is any leakage by looking the % moisture inside the coolant. If they check no sign of leaking but coolant level is reduce, it is normal, you can refill the coolant and test for 3 months. I refill my coolant after my car reach 5 years, nearly 50% gone, but no sign of leakage, they refill coolant for me and now adi 6 months the aircond still very cold. If leakage due to condenser, straight away change condenser + refill gas all those thing only cost you around RM600.

Aircond service is for those cars that doesn't need dashboard dismantle, definitely not our Saga.