I've given this a lot of tho't from time to time.
If the smith orders the parts and pieces from a supplier he should get the margin and I would even expect to pay the shipping and time for ordering.
If I order the parts, as I did for my current build in progress (bought everything from LRH posts of stuff for sale) the smith then quoted a guestimate for what special components that he had to provide (dies etc).
If a bit factored in for not making any margin on parts and pieces, so be it.
Whatever he does I have no way of doing.
Just had a new shingles put on the roof. A well known roofer, highly recommended by all, and just an ordinary man, scout leader, father, husband etc took a little under a week to getter done. Did a superb job. His labor charge was only 600 bucks. Hell, I wouldn't expect anyone to do it for that little when I could do it myself but am too lazy and old to get on the roof in 90* temps. I told my son, who is a tight wad, who had already written the check, to bump it a bit. Too late. He starts the siding this week, I catch up with him on the siding labor.
On the other hand, there's a socalled smith a couple of towns up the valley that took a total of 10 months collectively to install a brake then later swap barrels and reinstall the brake. To him I don't return and if I do, its for something like a quick bore scope task that I beat him out of.
Good wives, barbers and rifle smiths are way to hard to come by to not treat them right.