These prices are not out of line depending on the price of R-12 in your area. If your system is just low on R-12 have it charged up. The only time I recommend switching over to 134A is if the A/C system has a major failure, e.g. compressor seizes.
Be aware the EPA DOES care what happens to R-12. Should the technician find a leak and you decline to repair the shop is required by law to reclaim the system.
Fact of the matter is that ALL automotive A/C systems leak. There are too many moving parts, seals, hoses, fittings for them not to leak.