What should I pay for A/C service?

[DieselHead]

Hi all,
I just stopped by my mechanic's shop today to set up an appointment for him to look at and probably overhaul my a/c system. It's slowly gotten weak over the years to point now where it is basically ineffective. He was pretty hesistant to give me an estimate without looking at it so I wanted to know what you all have paid so I can know what to expect. The compressor isn't seized and there is no major damage to the system but I assume that some seals and things like that will need attention. Oh, and I am converting it to R134. I would really just like a ballpark $$ number so I'm not totally surprised when he tells me the damage because I am really in dark about what something like this should cost.

Thanks a lot

Alex