Basic A/C service consists or evacuating the system and the equipment should measure and hold the amount of refrigerant that is removed. The vacuum pump should be kept running after the system has been pumped down to 1" Hg. or less for several minutes, then the pump is shut off and the vacuum should be observed for several minutes to see if it holds. This is one leak test. The high vacuum will cause moisture absorbed by the receiver/dryer to be purged from the system.
If the system holds a vacuum, which indicates there are no leaks, the refrigerant that was removed is reinstalled and fresh refrigerant is added to bring the system to the 2.2 pound capacity.
Once the system is full the system should be engaged while the tech uses the detector to check for leaks. It's also a good idea to ask them to install the leak check dye, which will show up under a black light for future leak detection.
If your system is just barely operating, the refrigerant charge is probably down to about one-half, so if the their basic A/C service as described above is 40 bucks, and they charge 60 bucks per pound for R-12, and you need about a pound, you should get out of there for a little over $100.
I've talked to a number of industry professionals - many at SEMA with companies that sell A/C servicing equipment - and every single one of them has recommended that I stay with R-12 as long as it is available.