This is actually a compelling idea—I've been kicking around a similar idea for a while, where you'd like to buy something (e.g. a 2TB SATA drive) but only when it goes on sale below a certain price threshold. The problem is: what's the business model (i.e. where does the money come from)?
Have you checked out Priceonomics? They're a YC startup that gives you analytics on the market price of a product and send you notifications when the item is available for a good price.
I would say the whole thing is a monetizable business. To have good data, you'd need a mix of human and tech input. So why not charge $x for y number of updates? or charge for specific searches that requires more human work? or charge for a premium account that shows price fluctuations of certain items for the last 6 months?
You may be interested in http://camelcamelcamel.com/ for this sort of thing. Uses Amazon to track historical high and low price points so you can get a good idea of when to expect sales. You can set up a trigger to notify you when it reaches a certain price, too.
