Like someone else said, Apple Maps and I believe Google Maps now both use vector rendered cartography so they only pull down text & numerical data which has a very small bandwidth footprint to build the map images with. The data is just telling the maps what to look like and then they're rendered directly on the phone, whereas the old Google Maps would actually pull down the full street / satellite image tiles themselves directly from the servers, which swallowed up bandwidth. I'll bet you could get lots of maps use out of a cheap 500MB data plan.