You do not say where you are located. If you are in Texas or in another hot, humid area I expect you would notice the difference.
R12 goes for as little as $20 per pound now.
Converting a system to 134 in almost all cases is false economy. To convert properly, you must remove, empty and flush the compressor, flush all components, put in Ester oil, use 134 compatible o-rings everywhere, change pressure switches and expansion valve, evacuate and charge.
You should not convert a system unless there is a problem. If there is a problem it typically will cost much more than the refrigerant that goes back in.
In most cases you will not know the difference in refrigerant cost and by staying with R12 your system will in most cases last longer and in all cases cool better.
Have a great day,