This pretty much exists in at least Sweden. Check out http://www.prisjakt.nu/ ( translated to english http://translate.google.com/translate?sl=sv&tl=en&js... ). You said something like this would take 10 years to build and Prisjakt actually started 10 years ago. :-) They don't index everything of course but probably covers 99% of everything tech related and have been expanding in to new product segments all the time. Except the web site they of course do have phone apps which let you search for a product and then choose to display all the stores around you that carry that product and to what price and if they have it in stock. If you have a Swedish store selling anything online (or offline more and more) you "have" to exists on Prisjakt and have your full inventory with prices and stock status supplied to them. If you are among the cheapest stores on a popular product this of course will get you a lot of traffic. But there are more layers than just price indexing. They do have ratings and comments for all products and all stores. Myself personally would never buy anything from a store with bad reviews on Prisjakt, be it online or offline.

I would guess they started back in the days just scraping web sites on their own and then gradually got stores to supply them with "price files" that contained everything they needed. I know they have deals with create-your-own-web-shop businesses like Shopify and others so that these businesses create the price files automagically for every store THEY have.

As for revenue I believe that a store has to pay Prisjakt to actually have Prisjakt put up a clickable link to their web site. So if I search for Samsung Galaxy S III and go to the product page, I am presented with an array of stores carrying that phone, and I can click to go immediately to the stores own product page for Samsung Galaxy S III and go through with my purchase. If you don't pay it seems your website url is more hidden (on the store profile page) it's not clickable even their. You can also pay extra for having your logotype visible etc. They probably have a lot more going on as well.

2010 Prisjakt had $7.6M in revenue and made a $3.36M profit.