Thanks for posting this, OP! Yes, others are right that crowdsourcing is almost impossible because this is highly localized (even varying by neighborhood within cities) but it is TOTALLY worth it and I wish I'd started a grocery price spreadsheet ages ago.
I finally got around to it this January and it has already slashed my monster of an (organic) grocery bill in more than half. I found that Costco does not earn its keep for me except for very specific products. Now I know which ones those are and I'm debating whether to ever buy a membership again. For now I'm going to decline renewal and buy some cash cards to be able to purchase some stuff there next year and then reevaluate.
My approach to this problem is somewhat different. Here are the column headings of my spreadsheet:
1. Item (e.g. "almonds, sliced). I have a couple hundred items we buy regularly, listed alphabetically so I can look up good prices and sales at a glance.
2. Unit Price
3. Store
4. Date of purchase (to track sale cycles and seasonal lows)
5. Sale? (I check this when it is on sale rather than being a normal price point)
What this does is tell me what I should be paying ... the "buy price." I don't buy it if the price is higher than the normal low. It also tells me where to buy what, and in some cases when to buy what. When buying meat, poultry, and sometimes cheese, I now buy as much as I can in bulk when it is on sale below the normal buy price, and I then wrap everything into portions in freezer paper, label it, and freeze it. I now have chicken locked in at .79 or .89/lb for the next few months and I haven't even done this for a year yet! I'm pretty sure I will see lower. I shop Aldi for most stuff, then Costco for certain well-priced items, then Kroger only for what I can't find elsewhere. I love Publix' atmosphere and labor practices but I don't shop there anymore.
The spreadsheet took some time to start up but it is already paying me hundreds of dollars per hour of work! I started by just snapping pictures of shelf price labels on my phone and going through photos + my receipts. As I add to my file, I do not keep 10 different prices for almond slices, for example. I only keep the lowest normal price (which I now know is at Aldi here, not Kroger or Costco), and then I keep sale prices or seasonally variable low normal prices. Using this, I now know where to get my almonds and what I should expect at the register. I learned that Aldi offers many of my staple foods, especially poutltry/meat at about half the price I would pay elsewhere. I know to buy Costco only for specific items such as frozen wild salmon and frozen fruits and veggies like broccoli, wild blueberries, and cauliflower. Surprisingly, buying in bulk is NOT always cheaper, even when it comes to paper products. Costco produce packaged in bulk is almost never cheaper than Aldi's normal quantities here. Buying in bulk for perishables risks waste and precious fridge/freezer room, so this was a good lesson to learn that I need to do the math. Always.
I can't believe I didn't think to study prices sooner. Now, I don't go to the store without this file open on my phone. I don't shop what we want to eat tonight; I shop what is on offer now that is a good value and cook based on that. I still don't coupon, but I do look at Aldi weekly sale ads and pounce on low prices, especially for meat and fish. In the end, we are eating more seasonally, which means fresher food and fewer food miles which is an environmental win too.
Because I insist on organic and cage-free when possible for environmental/moral reasons, my grocery bills were totally out of hand last year, so I expect this to save us at least $5k this year. I highly recommend this method. There are a ton of bloggers that advocate this. Here is one article with instructions:
http://thefrugalgirls.com/2012/04/grocery-price-book.html