The majority of the content is contained in JSON files which have a really tiny file size (a few Kb). So this means that until you are downloading attachments or images of some kind, the data use will be very minimal. Based on the way that the systems offline capabilities are designed, if you are really concerned about data use then we suggest you open Scouts | Terrain for the first time while connected to a WiFi network. This means that the majority of all content (excluding attachments) will be downloaded to your device at this point.
In summary – very minimal data unless the attachment you are looking at has a large file size.