Is nitrogen used because it is cheaper than oxygen?
This is not exactly a joke, at least not directly. It is a reference to a blogger called The Food Babe who has offered up some laughable ideas - in this case opining on the health effects of airline travel. She famously complains that : "The air you are breathing on an airplane is recycled from directly outside of your window. That means you are breathing everything that the airplanes gives off and is flying through it’s mixed with nitrogen, sometimes almost at 50%"
For those who don't get the laugh-out-loud moment, it isn't the bit about how the cabin pressurization works, or even the bit about mixing the air with nitrogen.... yes, those are crazy goofy, but if you still are in the dark, give a quick google for the percentage of nitrogen in the atmosphere around you.