Think camelcamelcamel with price charts (and other aspects of the products) tracked over time and Google Shopping so you can compare the same product among stores.
Hoping to get the MVP out this month, been tracking data heavily since ~last September and currently have ~150k data points across about 37k products in 32 stores.
Biggest problem I'm having right now is matching up products across stores, since each store can name their products differently, some like to add random text to the titles "NEW!!!!", and data points don't always match up due to the batch based aspect of the products. I have a very basic matching system working, enough to extract (some of) the needed metadata from products and roughly match the products.
Current version is good enough for the MVP launch, mostly just working on cleanup / UI work right now before it goes out.