I am doing a freelance project that requires a 3d model of an existing space. It is fairly simple, but I after using several figures, I have arrived at this: $550 minimum flat fee, not hourly. 
It really depends on the estimated amount of time that you feel it will take you, and the “value” that you feel it presents to the client.
Easiest way to test is to determin the amount of time needed, then what you feel it is worth and still make money on the deal. Then when you propose the survice to the client say something like. "I have a cost estimate assigned, but before I share this with you, do you have an anticipated budget for this survice? Remember make the client think they are running the show, and pulling the strings.
I really appreciate your advice. You are truly…truly…truly…THE MAN!!