ovi maps is free. however if you download the maps from your phone service provider there may be charges depending on your plan.

you can download the maps on your normal internet using ovi suite and put them on the n900 (that way it will only use the GPS and it will not have to download any map data). However searching and routing has to be done online, so either do that we\here you have free wifi or you can use 3g (but charges may apply for 3g).

As far as i know there isn't any free offline routing software (well.. if there is i don't use it).

You can use Sygic maps (this is a once off payment for the gps app) and supplies offline routing and offline searching. all the maps are on the n900 so there aren't any data cahrges, this will only use the GPS.