Most plans allow 5 gigbytes (GB) per month of data usage. If you go over the 5GB you pay extra and it can get costly.
5GB equals about two 2 hour movies. The navigation applications use a lot of data because they are constantly reconfiguring the map and other data as you travel.
The good thing, once you access a WiFi site (RV Resort, Starbucks, McDonalds...etc) the data usage over the 3G or 4G cellphone network stops so you're not eating up your data plan.
We are heavy users and as long as we don't download movies we are able to stay under the 5GB limit.
Between the iPhone and iPad I rarely use my laptop or PC any more.