Alas I don't have numbers to provide you per se, but I do know that the data usage made by this app is fairly low. Still, the usage will vary depending if you're moving or not, how you're using the app (searching for a restaurant for example) and to a lesser extent the strength of the reception.
Note that there is an "offline" mode with Google Maps which allows you to download the map of a set area! With this you would have access to basic information but you would miss the live traffic conditions and updates.
Hope that helps