Depends on the smith, but it shouldn't cost all that much.
Basically all he has to do is disassemble the gun, install the trigger, test it for function and reassemble.
Another option is to grind the serrations off the trigger, or at least reduce the sharpness.
Back in the days before the easy availability of different triggers, grinding the serrations for easy double action was a popular mod.
Its at least "possible" to do this without disassembly.
Cock the hammer and use ties to tie it back.
Cover the entire gun with a heavy plastic bag, tin foil, etc to keep grinding dust out of the action.
Tape the dickens out of the trigger guard behind the trigger and everything around the trigger.
All you want exposed is JUST the lower part of the trigger were the serrations are. Use plenty of tape to prevent a "run away" from penetrating to the gun.
Use a Dremel tool or a flex shaft with a fine grinding cylinder to grind the serrations to your satisfaction. Either all the way off or at least smoother.
Polish the area with a rubber abrasive cylinder.
Clean the area up and apply lube to prevent rust.
Unwrap and you're in business.