I've actually been toying with this concept for a weekend project, essentially a quick 'n dirty node daemon process that checks prices periodically, for sites it knows how to. More often than not the data is just sitting there in JSON in the <head>, so you wouldn't necessarily have to scrape it either. (e.g. fossil)
Open source the project to let others send PRs for other sites...
Toss in a sendgrid key for emails and you're all set...