Earlier quoted context omitted.
You do have to pay for air if you care what pressure it is at. Non-Sherpas will demand a refund of their ticket price if provided air at ambient pressure at 35000 ft.
Lets do the math then: http://www.wolframalpha.com/input/?i=785+millibars+*+876+cub... Result: Each air change costs around .62 of a gallon of gas (assuming perfect efficiency). Or around $2.50 - let's call it $10 to account for inefficiency. In contrast it costs around $10,000 per hour (not including salary) to fly a large plane. Saying they don't circulate air to save money is ridiculous.
You can work this out my looking at an an airlines profits, and how many flights they did a year.