Broadly agree with this and the parent - but there's an interesting alternative way to look at travel metasearch. If you think of obtaining prices as being aki

Broadly agree with this and the parent - but there's an interesting alternative way to look at travel metasearch. If you think of obtaining prices as being akin to 'indexing', then what metasearch engines are doing is building up a search index for flights. And not all price sources necessarily charge for data (do Google pay to index Hacker News?) Flight prices become stale as do indexed web pages - and as long as the user doesn't pay a transaction price for any search activity, then you can re-check pages based on user activity - in the case of travel, re-check the price once the user nears the booking phase of the funnel. As noted though, sparseness of travel queries is certainly tricky! The 80/20 rule applies to some extent, and popular routes will be the ones which have the best price cache. Another disclosure: I work for Skyscanner (our core is a flexible travel search engine)