Well, that info isn't out there yet. Not accurately. Any number will be for the Edge network. We need 6 months of 3G usage to know what 'current' iPhone users use.
Here's a starting point then: what do most people use browsing the web on their home computers in a month? Most people will use their computers for web browsing than their iPhone, but will likely be visiting the same sites with both devices.
Does the iPhone have a cache? If so, I can download maps on wifi, then not have to worry about loading those when I'm using the GPS. Not to mention going back to the same site multiple times without having to download every single graphic every single time.
It may not be a perfect comparison, but it's a start.