The only fair way that I see is to establish your operating costs and charge by the hour.
With that being said there is only so many hours in the day so you are limited to how much you can make.
A lot depends on market conditions, does the customer recognize the quality over other methods? Are they supplying the drawings or do you have screen time to bill for?
That can be hard to put a price on.
Always more questions!
Good luck, Steve