Firstly, don't let him convert it to R-134. All the jerkoffs use R-134 now and they make a case to "convert". However, the R-12 is just slightly more costly than the R-134 so there is no good reason to do it. However, your mechanic may not have the gauges or be setup to install R-12, so he tries to convince you to "convert".

Take the vehicle elsewhere to a shop that can and will use R-12. See if they have it on the premises. I'll bet your mechanic does not.

With regard to the cost, you usually have additional parts such as the drier, expansion valve, and possibly the manifold hose in addition to the compressor. The total is typically around $550. Labor is at least 4 hours to do the job properly. 5-6 is not unreasonable. Most of them won't do the job properly and won't flush the system correctly. They want to keep the time down........you know........

Then, you get to do it all over again.........within two years. This is surely the case if you convert to R-134. I spent way too much money on a/c for the W123 when it was converted to R-134 and it failed within two years. Don't you do the same.