If you're planning on 'tethering' your iPad through your phone and using the iPad for chart plotting via cell towers I'm afraid your data usage will be through the roof! I bought a 'Bad Elf' brand plug in GPS unit for my non-G iPad. $95.
It has worked well without any cell tower assistance so far. I use it with the Navionics app.
It did fail to connect for a day once but I had my iPhone as a back up.
I also use a Bad Elf GPS with my IPad and Navionics and can recommend both.