The cheapest flight I have ever found is ZAL-ZCO (Valdivia, Chile to Temuco, Chile). Full fare on lanchile.com is 3,500 CLP (5.35 USD), plus tax minus 3% on-line booking discount, for a total of 5,200 CLP (7.95 USD).
That is so cool -- eight bucks total for a flight. Granted, it's only 78 miles (25 minutes block time), and there's only one flight a day, but that is a price that can't be beat.
Expedia.com says the price is $37 for a Y fare, but for some reason Expedia.com only has access to Y fares for all intra-Chile flights.
It might be an error (i.e., should be 35,000 CLP one way), but even if that's the case, it's cheaper than any other mistake fare that I know of when you include the taxes.