I certainly would work from White Gold sheet. The spring sheet 14K White offered above should be perfect.
If you plan a simple foldover design with plenty of space and a good spring action, it should hold anything from one or two folded bills, to a significant "was of cash", with no need to keep rebending the money clip. The natural spring of the design should last a lifetime, if it is not repeatedly bent. White Gold sheet is perfect for this application.