I THINK ATT charges $10 to add an ipad to an existing account. Dont remember the data plan but probably 2gb.
I run Navigon in the ipad and iphone and love it. They used to sell GPS in the states and were arguably better than anything else out there at the time, but couldnt compete with Garmin and TomTom. But, the Navigon app is way better than TomTom or Garmin, and I'm a huge fan of Garmin. Navigon was bought by Garmin a few years ago and sofar hasnt ruined them, but they seem to be way more popular in the rest of the world than the big two in the US with iOS maps for almost every country in the world.