Refits to 134a will work if your outside air never gets above the low 90's. For those that don't live in those sorts of frigid wastelands R-12 is the best and cheapest solution.

Assuming no existing problems you are looking at $400 - 500 for an honest shop. You'll probably need to track down your own R-12. It is readily available on ebay, craigslist, or even this web site every once in a while. The average price is a little under $30/lb shipped to you and you'll need a little under 3 lbs.

You'll need:

3 lbs. R-12
1 bottle mineral (NOT PAG) R-12 oil
Dryer
O-ring kit

and you might want to change out the pressure switches at the same time.

All of this is under $200 for the parts.

My local shop charged $150 to evactuate the system, test for leaks, and then to add oil and charge. If you budget $500 for the whole job you should be fine.

My daytime temperature right now hits 103 F on average with peaks at 107 F. The stock system works well in city driving up to about 100 F. At that point I've got to keep the fans on high to be comfortable. On the highway the system works better but as that is not in the concrete oven in the city the air temps are lower there and so I have not tested the system above 103F on the highway.