According to the U.S. Bureau of Labor Statistics, prices for airline fares were 19.33% lower in 2000 than in 2017.
Airfare experienced an average inflation rate of 1.05% per year between 2000 and 2017. In other words, airfare costing $100 in the year 2000 would cost $119.33 in 2017 for an equivalent purchase.
Average pricing changed by 8.63% in 2000, significantly above the yearly average change for airfare during the 2000-2017 time period. Compared to overall inflation for all items in 2000 (3.27%), price inflation for airfare was much higher.
Raw Consumer Price Index data from U.S. Bureau of Labor Statistics for Airline fares:
How to calculate inflation for airfare between 2000 and 2017
Start with the CPI inflation formula:
CPI in 2017 / CPI in 2000 * 2000 USD value = 2017 USD value
Then plug in historical CPI values from above. The CPI for Airline fares was 239.425 in the year 2000 and 285.717 in 2017:
285.717 / 239.425 * $100 = $119.33
Therefore, according to U.S. Bureau of Labor Statistics, the "purchasing power" of $100 from 2000 is $119.33 in 2017 (in the category of Airline fares).