You shouldn't have to worry about your limited data plan, you could store all of your music locally on an sd card and not ever have to stream anything. Streaming netflix or dish network like I do would of course eat through data pretty quick. I'm just over 12gb this month!
It's no different than what you normally use.
And just to clarify, in case you are confused, you are not using to connect to the AppRadio, it simply is using your hdmi out. That seems kind of obvious but having a wife who is a complete tech retard, I sometimes have to dumb it down (no offense.)