Billing basis for 3D work

I am interested in how firms today bill for their 3D work vs. traditional working drawings. For example, how would an interior design firm charge the client for 3D (VIZ, MAX, Vray, etc.) renderings? Any suggestions based on actual experience will be greatly appreciated.

[MrCATIA]

I bill at an hourly rate, based on the job scope. One rate for simple 3D modeling/drafting, and a higher rate for product design. Likewise for consulting, training, end-user support, etc. There are many other variables.