I don't think you are going to find the magic number, but it would depend on how specialized the project is, and second, what will the market allow. Your competition is going to dictate what you charge if it is an average project. One of a kind projects with some degree of difficulty will allow you to charge a little more. Price shoppers don't understand the difference between Domestic plywood, import plywood, melamine, 3/4 vs 1/2. The materials used can make a difference of 50% or more. In my area, I can get 3/4 China birch for $23/sheet, 3/4 Domestic is running $48/sheet. This alone can make the difference between getting the job and not getting the job.
I had a contractor call me last year and wanted to know if I could come over and look at a project he was working on. I had never met this guy in my life. I get over there and the first thing he tells me is, "We have had a couple other cabinet and trim guys look at this and said they couldn't do it". Ca-ching, Yes I can do it but it's not going to be cheap, was my response.
The main thing I do is listen and read the customer. Why did they call me? Was I the first guy in the book or did they call me because of quality and craftsmanship.
The job turned out very well and was profitable

My knee got sore bending the molding to fit around that hallway.