Actually, you are right that there was a discussion on this on the general Q&A thread. One poster on there (David Beach) suggested that it was indeed possible to pay cash in pesos for airline tickets but the conclusion of the discussion was not 100% clear, to be honest.

If it is indeed the case that domestic tickets can be purchased in cash without problem irrespective of residence, then it probably is the best course of action to take for a visitor I would have thought, as it brings the cost of tickets back to reasonable levels. Perhaps Gaucho100K could seek a clarification about this when he speaks to LAN Arg?