Too many variables, from "free" when all they have to do is tighten a connection, to a several hundred to re-sweat a connection to a thousand or more to remove and repair a copper coil.
Not including the initial service charge, and topping/refilling the gas.
I wonder why people don't ask them to. A lot of people think the gas is some kind of consumable and just ask them to top it off.
A lot of service guys do not actually do repairs, they do flat-rate services in a competitive market with no customer loyalty and most everyone wanting the cheapest possible service. What do you think the margin is for a guy with a truck, tools, a pressure washer and a second-man cleaning air conditioners for 500 a pop? If your car is not running, do you take it to the carwash to have them fix it?
For repairs, the customer also almost always wants a price before the tech knows where the leak is, which the tech cannot provide, or if they do provide a quote, they have to quote it high. And then the customer thinks they're getting robbed when all the guy does is get his sniffer out of the truck, find the leak and tighten up a fitting or two.
Buy your air conditioners from an authorized supplier that is close to your home and that also does service. Quit worrying about a few hundred baht here and there and give them your business.