Nokia Drive, a free navigation app for the Lumia Windows Phones, has offline maps for regions and whatnot, which users can choose to download depending on what maps they wish to use - preserving space. For the actual turn-by-turn navigation the network connection is still used throughout the journey, and this could prove to be expensive for those on tight data plans or are travelling abroad.
According to Plaffo, Nokia has announced an update for Drive coming soon that should hopefully fix this issue.
Nokia Drive will receive an update in the near future that will allow navigation even offline.
Source: Plaffo, thanks Antonino for the heads up!