Hello,

I had the exact same thing for my domestic trip in Chile in februari. Found fares on itasoftware for some 280$ per person. (a rather complex trip as well). When i called LAN their lowest price was some 600$. Even if my sister (who lives over there) went to the local ticketing office, the price was 600. On the phone i literally told the CSR that i knew there were places available. Yet she (and i tried again on a different number) could not offer me those fares, as her system didn't offer me that price.

Eventually i found the tickets for some 310$ pp on expedia.com.

The point is:
ITA nor expertflyer are wrong. The fare is still available. Try some online travelagents, or try to mail the ITA-results to an independent travel agent. (When my sister mailed those to LAN, they couldn't offer her that price..)

And on a sidenote .. ITA is great software! Thanks to keep it available for "guests".