OK, the cost of replacing the air in your tires with nitrogen is so small that it's nearly not even worth mentioning. It's certainly not a large enough sum as to make a significant impact on the decision.
The air we breathe, and the air currently in your tires, is already nearly 80% nitrogen. The remaining 20% is mostly oxygen.
It's not like you're replacing the air in your tires with tomato soup or something equally silly and equally so radically different. You're replacing it with what is mostly already in there to begin with.
Removing the oxygen and replacing it with nitrogen will eliminate the oxidation of the rims where the rubber seals are formed with the tire, it eliminates the oxygen breaking down the rubber (elastomer) molecule backbones within the tire and therefor significantly reduces the chances of separation and blowout. Manufacturers put antioxidents inside the rubber formulations when building the tires to help protect against the attack from the oxygen, but these break down and literally wash away rather quickly, usually within a few tens of thousands of miles. It's simply better to not have the oxygen and the water inside the tires in the first place.
The larger size of the nitrogen molecule will permeate through the sidewalls of the tire at a much slower rate and will result in the tire staying inflated at a constant pressure for longer periods. Under-inflated tires result in more rolling resistance, more friction, and therefor more heat is built up in the tire. More than 90% of all blowouts are due to under-inflated tires. Couple of pounds under-inflated, on a hot day, with a heavy load, there ya go.
Nitrogen, being a lower atomic number than oxygen, even with its large molecular size, is lighter than oxygen and does not expand and contract as much as oxygen does over the operating temperature range of tires. Oxygen (and the associated water and water vapor contained within it) can expand and contract during normal driving to the point that the tires are rarely at the properly inflated level, and when the tires are at high temperatures and thus higher pressures, the oxygen molecules permeate through the tire sidewalls at even greater rates, so that when you stop and the tires cool down, they will need to be reinflated after nearly every trip of any significant distance.
Nitrogen, on the other hand, retains a relatively flat and even pressure within the tire at operating temperature that range from being parked in sub-freezing temperatures all the way up to skillet temps of the Arizona asphalt at 3:15PM on August 17th. Plus, with no oxygen and water/water vapor in the tires, there's no water to freeze when parked at those sub-zero temps, allowing the tire to roll freely upon initial movement, which reduces or eliminated the stress on the inner tire walls that can cause premature cracks and failures.
Sooooo, because the tires stay inflated at the same pressure during a much wider range of temperatures, as well as nitrogen running cooler and being able to dissipate heat more rapidly than oxygen, your tires are nearly always at the optimal pressure, which means less rolling resistance which yields less tread wear, better handling, better braking, and better fuel mileage. And it rides smoother.
Unless, you have some particular reason for actually wanting that 20% of oxygen and its associated water inside your tires, this is not a major decision that requires months of thought and reams of research, particularly in light of the minimal cost. Just do it.
The longer you wait before doing it, the harder you'll have to kick yourself for not doing it sooner. You'll get 20-25% more miles on your tires (I have 50% more miles on mine, which effectively reduces the cost of my tires by 33.3%, no small sum at $200 per tire), and you'll see anywhere from 2-5% increase in fuel economy, with the most often reported number being 3.3%.
Even if it's only a mere 2% increase in fuel economy and there are no other benefits whatsoever, what's 2% more MPG worth to you?
What are you waiting for?
Slow and steady, even in expediting, wins the race - Aesop
