Ha ha, I don't think I would take it on, and you possibly could not afford it if I did.
Just read a woodworker's opinion on this:
1) Put a value your time - at least $50/hour (you're an artisan with hard-won knowledge)
2) You put in a 10-hour day when you're working so: $500 per day for your time on a project.
3) How many can you build in 10 hours?
4) What did the material cost?
5) ($500/Number of projects completed in a day) + material = Cost to buyer
Of course, you can refine this number by asking how much time you actually spent before you reached a point where you had to watch the glue dry, etc.
I think the formula works better on small part production than finished instrument but maybe this will give you a place to start from...
All too true Glen, I also consider cost of power,running machines, light, heating, or in my case cooling and at times dehumidifying. Also advertising, travel, phone etc., It might be a hobby but it has costs. But then so does fishing, trap shooting, motorcycling etc. with those though I don't have to deal with the tax man.