To get a truly convincing version of that effect, you would have to use some other application, as Keynote isn't a general animation tool.
That said, using the Move and Rotate actions, you could probably put together something that communicates that notion. I would use Move paths that gently curve and loop at different speeds for the different individual graphics, and combine those with Rotate actions so that as the pieces "fall" they also rotate in the plane of the slide (of course, you won't get any three-dimensional rotation using this approach).
Alternatively, you might just break down and use some stock video from a place like
iStockPhoto (search for "money" and "falling", restrict the search to video, and you will see they have many options available for very reasonable prices).