I used Excel to make up a simple form to type my grocery list into each week. (See below.) The first column is for the price, the second column is for the name of the item. At the bottom of the list are three rows: Limit, Total, and Kitty. In the Limit row is the budgeted amount for my weekly grocery list. In the Total row is the total amount of my grocery list estimated cost (more explanation on that in a minute). The Kitty row is the difference between the two.
As I write out my list, I estimate prices on things. It helps me pay attention to sale items. I can look at a weekly sale flier for a store and know if a particular item is really a good deal or not. I have the Excel spreadsheet set up so that it figures all the amounts in the bottom rows automatically, with the exception of the Limit amount, which I can change whenever I want/need to.
As I shop, I check what the actual price is, compared to what I estimated it would be. Any difference gets figured into or out of the kitty (which I write underneath or on the back of my list as I'm in the store – the typed kitty amount is just a starting point). For instance, two weeks ago milk was $1.66 a gallon. So last week I budgeted $1.75 per gallon (I always round up to the nearest quarter dollar). But when I went shopping, milk was only $1.49 per gallon. When I bought four gallons of milk for $.25 less than I'd budgeted, I added $1.00 to my kitty. If something is more than I estimated, it comes out of my kitty. And to keep things simple, I always round up to the nearest quarter dollar. At the end of my trip, I can either choose to spend my kitty on something not on my list, or save the extra for another time. Sometimes I make a short list of things I come across in the store that I'd like to spend my kitty on so I can go back and get them. If I'm under (which rarely happens), I decide on something to put back. I usually put aside a couple of dollars for taxes, but usually my rounding up covers any taxes.
On the second page of my "Groceries, Meal Planning" spreadsheet is a meal planner. I like to fill this in as I make my grocery list, then print it out. I post it on my fridge and use the bottom of it to write down things I need to add to my grocery list for the following week.
I'm going to try to upload the spreadsheet so you can snag it to use as you so choose. Happy grocery shopping!