Google Maps might be helping out lessen your cellphone bill with the new feature they're testing. They're trying a new Wi-Fi only mode to take the place of Offline areas. What this does differently is it will use smaller amount of data than offline mode. The existing offline mode stops using the pre-loaded amaps once it's able to connect to your mobile data, so unless you turn off the radios, it'll use up some of your data.
Source: Engadget