This is a common misconception. On-demand GPS navi maps do not use up much cellular data. The main point of having locally stored maps is when you are traveling in areas with really poor cellular coverage.
Trust me, I've tried probably six on-demand GPS apps, and maybe three GPS apps with local map storage. The on-demand GPS apps will cache map data, so if you travel a regular route, the maps won't need to be redownloaded. The majority of your cellular data will be things like live traffic. Some on-demand GPS apps can preload maps on a WiFi connection before you start your trip.
I really don't want to use up 1.5GB of storage when 99% of my road miles are within 100 miles of my house. I don't need an entire roadmap of the United States. I did buy the CoPilot Premium USA app (local maps) when it was on sale, just in case I ever need local maps. Also, there's a variety of NavFree apps (they used to have state-specific apps for certain states).
Trust me, I used to have an AT&T GoPhone SIM with Pay As You Go data packages, so I was very conscious of where my data usage was coming from.
These days, I generally use Waze (free, on-demand maps; free live traffic). The Garmin onDemand app is pretty good, a more polished UI than Waze, but a monthly subscription is needed after the original 60-day trial period is up. I keep it on my iPhone just in case I'm in a place where Waze doesn't cut the mustard. The MotionX GPS Drive on-demand app is a back-up. They might take up about 10-20MB each, not a big deal to keep them around.
Streaming video (and to a lesser extent audio) will chew up cellular data, as will uploading photos and video to Facebook, YouTube, Instagram, etc.