I am not sure how the system for most grocery stores works now, but in the ancient past there was a central authority that set the prices for individual stores

I am not sure how the system for most grocery stores works now, but in the ancient past there was a central authority that set the prices for individual stores (so in Australia, the Coles or Woolworths all have individualised prices per store for items except for advertised sale items). The computer in the store that drives the POS scanners then can print out the shelf label stickers and a poor sod goes and replaces the stickers on the shelves. This is all a big secret of course, so asking a grocery store for their prices is going to get you nowhere. You could try scraping the websites, but often the price for online shopping is different to what is in store. Best bet would be to offer an incentive for people to scan their receipts into your app. I don't know what kind of incentive you could build to do that, maybe for every 5 receipts you could offer a coupon or something. Not much of an idea and a massive PITA, it would also only give prices on those things that people bought. edit: also gets complicated for multi-item discounts or combination discounts