The cheapest international destination on low-cost carriers was China ($1.22 per 100km), followed by Romania ($1.69), Malaysia ($2.11), Tunisia ($2.78), and Portugal ($2.80). The most expensive international destination? Canada, at $43.70 per 100km. Maybe that Montreal bachelor party actually would be cheaper in Lisbon.
For domestic trips, India was No. 1 at $2.27 per 100km on low-cost airlines. Malaysia ($2.32), the US ($3.54), the UK ($3.78), and Portugal rounded out the top five. The most expensive was the UAE at a staggering $181.38 per 100km for domestic flights, an enticement to walk in 120-degree heat. The API also calculates costs for legacy carriers, a department in which the US climbed to No. 2, at $4.82.
If you want to see how all 75 countries ranked for international, domestic, and average airfares, and comparisons between low-cost and legacy carriers, the full study is available here. It has handy sorting buttons that'll do the comparing for you, so you can figure out where your airfare dollar will go the furthest. The answer, however, may very well be that is goes the furthest at home.