1) CO2 absorbs certain infrared wavelengths. You might consider checking your laser's output against CO2's absorption spectrum. It might make your cuts a little fuzzy if they coincide.
2) Are there cheaper gases? If the goal is to displace oxygen, even the product of a gas flame (which is mostly CO2 and N2) might work. Implementation is left as an exercise for the student. (Although Scientific American ran a piece decades ago on how to make low pressure N2 for a homemade UV laser from regular air with a Bunsen burner and some plumbing that would have made Rube Goldberg proud.)