The only way I get GPS on the AppRadio 3 is by enabling the cellular data function on my iPhone.  I don't web-surf on the cell phone except on my network or in a pinch, and usually keep "cellular data" turned off.  I don't have a feel for how much it may cost me to use GPS.  Or is it just pinging the cells.  I don't want a shock from my next cell bill.

I don't know the answer either (I'm guessing it's minimal). 

http://www.lmgtfy.com/?q=How+much+of+my+data+allowance+is+used+for+GPS

I'm learning as I go.  I can use Navmii, a free app, that needs cell no connection, but is IMO weak as a nav app.  Or with MotionX GPS-Drive, a much better nav app, I can can preload maps for a trip on wifi at home, and the only cell usage while driving is for route calcs, and is pretty low.  I hope other alternatives will arise as more nav apps hopefully become usable with AppRadio.

You can also use Google Maps and pre load the maps... If the HU is Using GPS it shouldnt use data (GPS is not a cell company feature and is not paid to use feature) but AGPS is a Feature that will use data... GPS verse AGPS

With AGPS instead of Global Satellites as location means it uses Cell phone towers (2 for low res location, 3 for accurate) those cell towers are you cell companies towers.... AGPS is a faster lock on phones but does use a little data... Phones mostly use this as a handoff get a quick link via AGPS the handover to full GPS when a lock is acquired.

AGPS- Assisted Global positioning System - Uses Cell phone towers and data connection to find location (carrier may charge against data plan or air time minutes, very fast position locator

GPS- Global positioning System - Is a chip inside of phone there is no carrier charges has slower connection, can make phone hot, Chips in phones hasn't been upgraded in phone in a long time (like 5 to 10 years)