Mitch,
When I looked into an R12 to R134A conversion for my 1987 XJ6 Vanden Plas offered by a local independent shop it was more than $2,000 (although I don’t have the details anymore). I bought a vacuum pump, an o-ring kit, and an A/C manifold setup from Harbor Freight. Then I removed and replaced all the old o-rings and did a vacuum check only to discover that the compressor had a leak. But with a rebuilt compressor the car held 30 in Hg vacuum for a few weeks while I worked on other things on that car. Once that car was back on the road from my other work, I removed and replaced the receiver drier and had it professionally serviced with R12 for about $250. A year later it is still blowing cold in the hands of its new owner.
Following my success with our 1987 XJ6 Vanden Plas I followed the same method to get our 1990 V12 Vanden Plas and most recently my 1984 XJ6 Vanden Plas blowing cold with R12 again. I have two more Jaguars to fix the A/C on, but they will have to wait for now while I complete other work.
At this point I got three cars blowing cold with R12 for less than the R134A price quote that I got for one car. It was a lot of hard work on my part, and there are many potential sources of leaks that need to be tested and fixed (o-rings, compressor, condensor, hoses, fuel cooler, evaporator, ?). But overall I found doing some of this A/C work myself more satisfying than throwing more money at others only to be disappointed yet again by warm air shortly afterwards and money down the drain (or into the atmosphere?).
I learned some new things about how A/C systems work (and why they don’t work) plus I got to buy some more fun tools for my garage. 
Paul