, not really afaik, but you can just test how much google nav uses one day and then use the method above and that will give you a rough idea.
I thought it would have been a fair bit more than that! 
Also you can precache up to 10 areas IIRC. So 10 miles within each direction of certain places, means no need to dll any data at all, but I think this is only for maps and won't apply to the navigation as well.

You'll have to make sure ALL the route & side routes are precached otherwise you could end up with a nasty bill....
AND
If there isn't phone coverage along your route......
This is why I spent money on Co-Pilot![]()
Swype should do stats (or in my case, HTC keyboard) It saves loads more keystrokes than that....![]()
Symbols board to get carriage returns yo! Although the new beta changed to always carriage return
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Screenshot saved, upload and reply posted all via the phone. Android for the win!