How much does it cost to convert your car ac system from r12 to r134?

Depends, if you're going to keep the rig for more than the next 3 years, stick with the R-12. Long term reliability of R-12 systems is much better than 134a systems. The mineral oil used in R-12 is much less reactive than the POE or PAG oils used with 134a systems. The money you save on the 134a refrigerant costs will be offset by the parts that you must replace to do a top quality 134a conversion, i.e., new compressor, barrier hoses, new dryer, etc.... Bottom line, stay R-12 if this is a keeper..

R.K.