Comparison shopping within a store is pretty easy with a calculator in hand. However, unless you store all your prices in a spreadsheet, this task can be challenging because quantities for even standard goods may differ.

Case in point: Brita water filters. Costco sells a 10 pack, Target sells a 6 pack, and Wal-Mart sells a 4 pack. Wal-Mart sells a 4 pack of Brita filters for $19.44, or $4.86 per filter. The best price from my last price check was $4.199 per filter from Costco. However, Wal-Mart did edge out Target be a few cents per filter (i.e., $4.86 v. $4.948). However, I spotted a 5-pack of Brita Water Filters at walmart.com selling for $39.98, or $7.996 per filter! The lesson here is to not assume that the in-store and online prices are the same, or that online prices much be cheaper than within a bricks and mortar store.

This exercise in tracking the cost of various products that I usually purchase has really focused my attention on consumer prices. I know that gas prices change daily, but what about the price of other consumer goods?

Since the last post on Bona Hardwood Floor Cleaner, I noticed that the Costco had changed its price. Since the schools only teach about inflation (and not deflation), you may think that prices will only go up absent a sale or rebate. But, there I was looking at the sign and sure enough, Costco now sells Bona Hardwood Floor Cleaner for $11.99 instead of $12.69. No sale. No instant rebate. Just a straightforward price cut. Wow!

Target sells a 30-count box of Refresh Plus single use vials for $6.99, or $0.233 per vial. Costco sells a 100-count box of Refresh Plus single use vials for $18.39, or $0.184 per vial. If you use a vial a day for an entire year, Costco will save you $17.89. That’s IF you use a vial a day.

Pay attention to the quantity, not just the price. At first glance, I thought Costco was selling Kokuho Rice at a higher price than 99 Ranch. After all, a 20 lb. bag of rice doesn’t look too different from a 25 lb. bag of rice, especially when both bags are not next to each other.

So, Costco is selling a 25 lb. bag of Kokuho Rose Extra Fancy Rice for $20.99, or $0.840 per pound.

99 Ranch sells a 20 lb. bag of Kokuho Rose Extra Fancy Rice for $16.99, or $0.850. So, for 20 pounds of rice, 99 Ranch charges about 20 cents more for the entire bag of rice. Not a whole lot, but still more than Costco.

Costco regularly sells a 15 lb box of Oxi-Clean for $16.99. Currently, Costco has an instant manufacturer’s rebate of $4.00, which drops the price down to $12.99. So, if the sales prices at traditional retail stores are almost as good as the regular prices at Costco, how good are Costco’s sales prices?

Mind you that this is not a mail-in rebate or one that requires a coupon in hand. The discount is automatically taken at the register. Since, 1 pound = 16 ounces, Costco sells Oxi-Clean at $0.071 per ounce (regular price) and $0.0541 per ounce (sale price).

Target sells a 6 pound tub of Oxi Clean for $8.49, marked as a temporary price cut. The regular price is $9.99. So, Target sells Oxi Clean at $0.104 per ounce (regular price) and $0.0884 per ounce (sale price). Currently, the OxiClean site has a $1.00 OxiClean coupon for any OxiClean powder (56 oz. or larger). Since 6 pounds = 96 ounces, the tub qualifies. So, at $7.49, the unit price is now $0.0780 per ounce, which isn’t even better than the Costco regular price. The killer deal is that the Costco sale price is over 30% the Target sales price + coupon! For the same amount of stain remover, 6 lbs. at the Target sales price with coupon is $7.49 and 6 lbs. at the Costco sales price is $5.19.

Granted, whether you are buying 6 lbs. or 15 lbs. of Oxi Clean, either size will last a long time. While you do save $2.20 per 6 lbs., saving a couple dollars a month isn’t the same as saving a couple dollars per week (or per day).

Costco sells a 70 ounce box of Kellogg’s Frosted Mini-Wheats for $8.49, or $0.121 per ounce.

Safeway sells a 24 ounce box of Frosted Mini-Wheats for a new low price of $4.49, or $0.187 per ounce. Wow! So, Costco is saving me $4.62 per box, compared to the Safeway per ounce price.

If I consume 2×70 ounce boxes per month, my annual savings is $110.88.

Costco sells a 10 pack of Brita filters for $41.99. The math is really easy for this one. $4.199 per filter.

Target sells a 6 pack of Brita filters for $29.69, or $4.948 per filter. If you change your filter every two months, that’s six filters per year or an annual savings of $4.50 by buying in bulk from Costco.

Costco sells an expanding mix of Asian foods. In addition to Ling Ling potstickers and jasmine rice, I have also seen edamame, shrimp won ton soup, and steamed pork buns. During my latest visit, I spotted a 32 ounce bottle of Lee Kum Kee Premium Oyster Sauce for $6.89.

So, how competitive is the cost of Asian food at Costco compared that from an Asian supermarket?

99 Ranch sells an 18 ounce bottle of Lee Kum Kee Premium Oyster Sauce for $3.99, or $0.222 per ounce. The Costco per ounce price is $0.215. To be fair, I should be looking at the price of the 32 ounce bottle at 99 Ranch, but I could not find it stocked on their shelves. I could only find a large bottle of the LKK Panda Brand Oyster Sauce at 99 Ranch.

99 Ranch sells all varieties of Berkeley milk for $3.79 per gallon.

I can taste the difference between different brands of ice cream and yogurt, but milk is pretty much milk to me. So, one gallon of homogenized milk for $3.79 at 99 Ranch or two gallons for $3.99 at Costco? Even if you cannot finish a gallon in a week, two gallons at Costco seems to be the way to go.