Man.. are you serious? nitrogen is free?
I think nitrogen is still kind of rear in GreenBay, or Northeast Wisconsin as I remember I watched a news coverage regarding to Nitrogen use over here and it charges for sure over 10 bucks for it.

I think those $10 is a one time charge and it is for a life time deal.

Regarding to the nitrogen in hot pavement, it makes sense that it doesn't compound as much as usual air. However I am more concern in winter time which the air pressure will drop. Nitrogen is less responsive, or hold more constant pressure in a bigger temperature difference, and that's my primary reason for putting nitrogen in..