Off-board, cloud based cell phone GPS navigation takes around 1 to 15 megs per trip, with wide variation based on pre-caching, length of route, and other unknown factors.
A limited test on iOS shows Apple Maps and Google Maps to both have under 0.5 megabyte data usage for most navigation steps.
It seems on iOS, Apple Maps has a less data usage compared to Google Maps. However, it is not a large enough difference, in my opinion, to affect most users.
Scout by Telenav has consistently higher usage, but again it is debatable if over a month if it would add up to enough to affect any users.
The main item to note is if a user wants to save mobile data usage, preplanning and letting system precache data while on a WiFi connection is a worthwhile way to save a megabyte or two of data usage per trip.