I thought about trying to do this for something like groceries, but grocery stores don't list all their prices in an accessible manner.
I don't think they want to, either. Every grocery store, I'm sure, calculates how much they can discount some items to draw you into the store so you pay exorbitant prices for others. In the end, they profit. If all prices were accessible online, it would be possible to write an application to minimize consumer spending, but this would cause the whole system to collapse. If all consumers could get all the deals everywhere--throughout their whole shopping list--the grocery stores wouldn't profit.
But is this really how people use grocery stores these days? I just can't picture people running around to different stores, trying to get the best deal on broccoli.
If there is one big grocery store significantly closer to my home than any other, that's the one I will go to when I'm out of food, no matter what. If it's late, I'll go to the one that's open late. If I have options, I'll go to the "cheap" grocery store, which tends to have better selection and slightly lower prices, on average.
Never, ever have I done prior research on where to go or what I was buying, except to check their hours. Most of the time I don't even know what I'm going to buy.
So, they can't lure me in with sales. But what they could do is keep track of which flavor of yogurt is ten times more popular than every other flavor and stock the shelves proportionately, so that I have a fighting chance of actually being able to buy most of the things I want on any given trip, and so they don't have to throw tons of unpopular food in the trash.
They could also lay off the guys that awkwardly try to pack food in my backpack for me since I can do a much better job of that myself. They might as well lay off the cashiers too and replace each one with two or three of those lovely self-checkout machines (I have tremendous sympathy for young people who find themselves unemployable, but doing work that doesn't need doing is no solution to that).
Because of the location thing, the grocery stores aren't really competing with each other for my business. But they are competing with restaurants and starvation, both popular alternatives to the hassles of buying groceries.
If e.g. Google launched a grocery store chain, with all the expected technological and common sense enhancements, they would steal most of my business and create a lot of value that was never there before.
I don't think they want to, either. Every grocery store, I'm sure, calculates how much they can discount some items to draw you into the store so you pay exorbitant prices for others. In the end, they profit. If all prices were accessible online, it would be possible to write an application to minimize consumer spending, but this would cause the whole system to collapse. If all consumers could get all the deals everywhere--throughout their whole shopping list--the grocery stores wouldn't profit.