I don't know all of the ins and outs because I am still learning myself,
but I fly with 2 guys who LIVE and SWEAR by this App.
One told me that the App download is free but the subscription to the service costs $75/year for the basic and $150/year for the advanced capabilities (which includes the little airplane symbol tracking along the moving, approach plate, airfield diagram, etc...)
I have not yet seen this is practice yet, but I'll be getting his iPad pretty soon and I plan on subscribing to the service myself.
If you find out more details, please share.
USMCFLYR