There is only one item that I diligently track the per unit price of and that is diapers. I still have one child in diapers and we have gone the disposable route. I did cloth diapers when both of my girls were infants. I was given a set of infant cloth diapers as a gift, but when they outgrew the cloth I switched to disposable.
To save myself time and money, I have found that buying Luvs diapers at BJ's (a warehouse club) has been the cheapest for me. I tried BJ's brand diapers and I did not like them. I liked the price, but did not like the diaper. They smelled terribly of plastic and they bunched up. So I stick with Luvs, because they are cheaper than Pampers or Huggies and BJ's usually offers a $5.00 coupon off of a box. That brings the price down to $24.99 a box and my baby wears size 4. With 180 diapers in a size 4 box that makes the price per unit of .139 per diaper. I use that price as my gauge and if I find them cheaper elsewhere I will buy, but BJ's is where I usually shop for diapers. That box will last me a little over a month.
I was thrilled when I read at Common Sense with Money about a Huggies diaper deal at Rite-Aid. I followed her instructions and I bought 5 packages of Huggies for a total of 150 diapers and a price of $57.45. I used 5/$1.50 Huggies coupons. I made 2 separate purchases and used a $5/$25 purchase coupon on each purchase. That brought my out-of-pocket expense to $39.95, but through Rite-Aid's Single Check Rebate program, I earned a $25 gift card for Rite-Aid and a FREE voucher for a package of Huggies diapers.
My final per-unit price will be .08 per diaper. I was thrilled.
Outside of diapers, I don't really track the per unit price of other grocery items. When shopping, I will look at the best price per unit on the items that I need on that particular trip, but I don't keep a price book. There are other items where I think a price book would be helpful, like coffee, toilet paper, chicken, turkey or cereal, to name just a few. These are constantly on my shopping list, so figuring out a "buy" price would help me know when I find a sale if it is a good time to stock up.
I have been working hard trying to reduce our grocery expense. Instead of a binder or book, I am going to write my "buy" price for our main grocery staples or essentials on a 3 X 5 card and keep it in my coupon file. I won't feel so overwhelmed and I think it will help me save some money by knowing when to stock up.
Do you track the price per unit when buying your groceries? Do you have a price book to manage your "buy" prices?