What would be a fair price for a revolver?
I think the only fixed price would be for a customer prepp"d firearm or one that doesn't need preparation. Otherwise I think you would have to see each firearm to determine how much time you would need to invest in preparation. Defarbing and fixing gouges with a file takes a lot of time to bring back the original lines. Surface scratches that can be taken out with 320/400 grit sandpaper less. But each one would need to be evaluated.
Maybe you could offer a fixed price for a 320 grit finish with the caviat that scratches deeper than 320 would remain. At that point it is a matter of communicating with the customer what they will get back for the price quoted. So to spitball a price list, and these aren't based on what you want for your time but a starting point aimed at undercutting established services prices. Only you can determine what your time is worth.
Repro revolvers
Customer prepped/no prep required revolver - $75
320 grit finish revolver - $100
Deep scratches/ gouges revolver - $175
Defarb revolver - $200
If your sticking with repro revolvers, the soft steels that make up the barrels takes rust blue well and could take as few as 3 passes. Cylinders, and I have to assume receivers, take a few more passes but still color well. Modern barrel steels take a lot more passes and anything like nickel steel start to be problematic as far as how many passes and what color they come out. Hardened steels are flat out difficult to both prep and rust blue much like the nickel steel barrels.
Sticking with repro revolvers you might want to think of it as
Rust blue - Saturday and Sunday, not full time though
320 grit finish 4-8 hours
Defarb/deep scratches/gouges pretty much anything that requires a file - a full Saturday/Sunday minimum
Shipping - at cost
I think you would have to undercut the established services as far as price goes. So a repro revolver getting a 320 finish could be done on a Friday prep, Saturday/Sunday blue. And you would be able to do other work on Saturday/Sunday. How much is your time worth for that investment of time.