My rule of thumb
I have a “rule of thumb” that I use when I do not have any better information. It is a SWAG (silly wild ass guess) I have found in many projects, whether it is carpentry, glass, or whatever, that the materials seem to cost about half of the whole project. Turning that around, you could say, that if you don’t have any better information, think about charging twice what the materials cost you to make the item. Of course, if you have precise information about your particular circumstances, that is much better, but this works in a pinch.