Highly likely that the evaporator or cooling coil that sits behind the dashboard has accumulated a layer of dust/grime over the years thus creating an insulating layer preventing the air from the fan/blower from being cooled down to the actual temperature as the air passes thru the fins of the chilled evaporator.
The RM280 quoted involves removing the evaporator from behind the dashboard and giving it a good wash to remove all foreign matter that has been caught in between the cooling fins, I have seen apart from dust, hair, feathers, scraps of tissue, dried up chit-cak and what-nots,
that's why the car's floor mat needs to be regularly emptied to prevent the circulating air within the car from picking up dust and depositing them on the cooling fins and over time reduce cooling efficiency.
Installing an air filter (which is not standard in the Myvi) at the air intake of the air con blower actually helps to filter out the pieces of foreign matter and helps prolong the cooling efficiency. Cleaning or replacing the air filter regularly is easy and cheaper than having your dashboard dismantled. Also physiologically you will hear creaking noises from inside the dashboard while driving which you will swear that wasn't there before you had the air con serviced Car air con in-dash evaporator