The price will be determined by many things. I would expect that some shops might just extract the R12, replace the oil, evacuate and recharge. This would be the absolute minimum procedure that you should expect.
Others will insist on a filter drier, flushing of the entire system, oil replacement and recharge.
I would expect the first to be about $150 if the system has no leaks or other ills that need to be dealt with. I wouldn't want to guess at the second one.
The problem is that, usually people do not change to r134 because they woke up one day and thought that it would be a neat thing to do. They are usually doing it because their a/c no longer makes the car cool. This means that there is a leak or other ill that must be repaired at the same time. This means that it is impossible to know what it will cost.
The a/c on my Vette stopped cooling a few weeks ago. I had already charged it with R12 and put in flourescent dye when I did. When it stopped cooling the second time I looked for the leak with the UV lamp and found a $10 pressure cycling switch leaking like crazy. I removed the R12, replaced the switch, put in Ester oil, evacuated and charged. It now works great. The switch, oil and r134 probably cost me about $28. I don't know what a shop would have charged. I expect that they would have had a few hours in it, so probably about $150 labor plus material. If I were them I would have replaced the filter/drier if it were on a customers car, so that would've probably added $50 or so.
In the case of the Vette, I was lucky, it could have just as easily been the compressor or evaporator leaking which would add no telling how much expense. So, you never know.
Hope this long winded explanation is helpful,