iPhone hotspot as a 'router' ?

Hi,

Here's what I want to do...

I've tested that I can connect an iPad and a AirPrint printer to my iPhone's personal hotspot. So, that communication should be just going from the iPad to the printer (through the phone), not over the Internet.

Would that data/communication chew up my mobile data plan? I don't know if the data is just keeping "local" or if it has to go out to AT&T and come back and I get charged for that?

It's a GREAT way to connect devices ad-hoc, but not if I'm getting charged the data (even though it shouldn't be using "Internet data" really..... right?