It's a rather varied price depending what is checked or repaired (if anything) and how much refrigerant that is actually put in.
Some mechanics will simply add a can or 2 of refrigerant, and if cold air comes out, they are finished.
But understand that if your a/c system is low on refrigerant, that you have a LEAK somewhere. If the leak is not found and corrected, you will be low on refrigerant again, maybe soon.
The most common leak location is the compressor shaft seal. I would replace the seal because I know how, I have the special tools, and I know where to get a new shaft seal. But the average a/c technician will replace the compressor to get a new seal. This creates new problems and is expensive.
Other common leak sources are injury to the front of the condenser from road debris, damage to refrigerant hoses, and o-rings at connection points that split or just leak.
-Engine overhaul mechanic and general automotive mechanic since 1972.
-Automotive a/c repair since 1980.
-Retired in 2018.