Let me throw in my two cents because I just had my 1991 300E AC recharged with R12.

There is a mandatory check for leaks before recharge so that was $40. Indy said the GOV won't let them just pump in R12 into the system.

The system was low on R12 via pressure check. There was no noticeable leak. However, the Indy insisted on

1) recover the R12
2) evacuate the system
3) weigh how much R12 was recovered
4) fill the system with capacity which was 2.5 lb
5) put in UV dye.

Mine was low by 1.5 lb and the R12 alone was $84. The recovery/evacuate/fill was another 1 hr of labor ($65). So my total bill was $206.

I went back for the free black-light check and there was no leak.

I do not know if the recovery/evacuate is completely necessary. He might be able to fill some R12 based on pressure but he said the way he did it was the right way.