I just got a set of new Michelins and West Coast Tires charged me $5 a tire for lifetime Nitrogen. Since I always check my tire pressure when I fill up (or used to), I wouldn't have gotten the Nitrogen if it cost much more than that.

The trade off is that I have to trust that Nitrogen actually holds its pressure because I don't want to be testing it all the time and letting the air out. Yes, I could just take it back and have it topped off, but a hassle is a hassle.