User canvia posted information about this in another tree [0][1]. I think a good summary would be decreasing prices up until about 2003, when they seem to level out around the high $300's, with an apparently exceptional drop in years 2009-2010.

[0] https://news.ycombinator.com/item?id=8752643 [1] http://www.rita.dot.gov/bts/airfares/programs/economics_and_...