We do conversions at my shop. Its $210 total, OTD.
R12 is $85 lb. R134a is $28 lb.
I have a tiny tiny leak that causes me to ad 4-5 oz each summer, once a year.
I converted mine 3 years ago. I live in the Mojave Desert where all summer it gets 90*-110*. The a/c works good.
I have the 2 little factory a/c fans, and a 14" electric (1100 cfm). (no water pump fan)
R12 works best(colder in stop & go traffic, or doing in and out errands).
R134a works just fine after you are in the car for 20 min.
It needs MORE air flow over the condensor to remove the heat.
I am considering upgrading my 14" fan to an 1800cfm one.(same 13 amp draw)
That should help alot for town traffic.
ALL a/c systems will leak eventually. The freon molecules are smaller than 02, and find a way out. The system is under pressure.
When a shop does it correctly, using the proper machine, the second thing they do (functional test first) is suck out the old freon, and measure how much came out. Maybe f you are only a 1/2 or 1 lb low, with no obvious leaks, you might want to put R12 back in to get the coldest a/c.
Here is a little something I did right after the conversion 3 years ago. It helps while sitting at long red lights.
I diconnected the hose right at the rear window washer motor,
(under the rt. front inner fender, and ran a new hose up in front of the a/c condensor. I then 'T' d in 4 adjustable yard misters (plant waterers) to spray water onto the a/c condensor when I turn the knob for the rear washers.
It lasts about a week, then I refill the washer bottle.
I can actully 'feel' the air coming out of the vents get colder. I also bought a mechanics pocket thermometer and placed it (cover and all)into the center dash vent so I can keep an eye on the air temp.
It also tells me how hot it is inside my car when I get in it.
Its been 160* many times in the summer when I have to lock and leave it. And thats why R134a is not as effecient. All the upholstery and interior of the car is so hot, that it takes a long time to get it cooled down.
If you are running errands around town, it may never get as comfortableas R12, until you get moving over 40mph.
OK...done.