Shouldn't this also be separated between Navigation apps that use data (so if you are using it for example, on vacation in a foreign country, your roaming data fees could kill you), and dedicated Navigation apps that use the GPS chip inside the iPhone (and therefore do not use any data).
I have an old ipaq with a dedicated GPS chip and I use iGuidance software that loads up the maps into the ipaq. It is an old device, but when I go on vacation, I use it in the rental car for turn by turn spoken navigation and I do not pay any data fees, just the 1x purchase price. A friend of mine used his iPhone and maps to get from the airport to his hotel and his data fees were through the roof.