Write down the 20 items you buy all the time. Next time you're in each store you buy groceries/household items at, take a couple extra minutes to write down the prices of each item (note sale and "regular" price so you know which is the price you can count on every time.)
You can use this info however you like; for instance:
Add up each store to see which is cheaper overall.
Start making a divided grocery list by store so you can get the cheapest possible prices.
Figure out if/when/how often the gas cost to an out of the way store is worth it.
...and so on. Knowledge is power!
If 20 items is too hard, think Milk, Bread, Eggs. Price each of those according to the brands you use. I started that test about 10 years ago when I bought just those 3 items from Albertson's and it cost me $12! This was before organic was available!

When we were house hunting out of state last year I quickly ran into the 3 grocery stores in town to check out their milk, bread, and eggs prices. Cheapest to most expensive:
Walmart, Fry's, Safeway. And now that I've lived here 8 months I've seen that that's exactly how their prices run on everything!