A/C that may have a leak is not the best thing to be playing with. Most shops have a machine that they hook up to your a/c and it will pull all your R134A out and check for a vacuum leak at the same time. The machine can then fill up your a/c. Most places in my area charge about 50-70 for the check and then a per pound charge to fill up. If you don't have a vacuum leak and need other repairs it should cost you in the 80-120 area.