I agree--it really depends on how much $$ you have to put into a cake, the time, energy , etc., all need to play a role. The different fillings and icings play a role. The "extras" like cake boards and hardware need to play a role.
I went to the store and actually got the prices of everything I need to make a cake. I figured out how much each item was per Tbl, t., cup, etc., and made a pricing matrix from there.
It's really not all that hard to do. I just wrote down a list of everything I used and then when I was on my regular shopping trip--I jotted down the prices. I took them home and did the figuring.
Now--if someone orders a cake--all I have to do is look at my sheet and I know how much to charge.