It is free. The fact that your _cell phone operator_ charges you money for data has nothing to do with the phone or the application itself. Blame your operator for that. And it does navigation. Maybe it does not do it the way YOU want it to, but it still does do it.
And as I said, if you have downloaded the maps to your phone it actually doesn't use the connection for anything even if it is open.
even when it has been announced beforehand that N900 has been designed for being online 24/7?