Is there a generic page scraper library or toolset? I can see a ton of demand here and it seems like something you could use on your day to day. Things like this actually seem like an outlier, I'd imagine airfare would be a huge one.
For airfare, I think there's already a few players in the "this flight got cheaper" space (Kayak and Hopper come to mind), but they don't have data on some airlines (because of the airlines' ToS IIRC).