Costco does nitrogen empty/fill for $11.99 on all 4 tires. I also asked my local Costco tire tech yesterday about bringing my car in for rotation and balancing even though I didn't buy the tires from them. He told me he can rotate, balance, and nitrogen fill all 4 tires for $21.99. My local Honda dealership wanted $39.99 just for rotating and balancing.I never considered this but it makes sense and called around in my town at Discount Tire Centers which carries 100% nitrogen fills for $8 per tire. I'm going to replace O2 with N2 to test it out on my Insight.