I have 78% nitrogen in my car's tyres - it's free.

Mine was included in the price of fitting at Costco, so also free - wouldn’t go out of my way to have it done. N2 molecules are larger than the other molecules that make up ambient air, but they still escape via the valve when incorrectly adjusted to the wrong pressure.
 
I have 78% nitrogen in my car's tyres - it's free.

Not really the point here, if there was no benefit from using dryish nitrogen, then no tyre places would use it and no one would provide the kit to use it, but there are tyre places that use it and kit is supplied to use it so why not.