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.

Last month, I compared the price of Frosted Mini-Wheats at Costco and Safeway.

This past weekend, I stopped by Wal-Mart to shop around. Wal-Mart sells an 18 oz. box of Kellogg’s Frosted Mini-Wheats for $2.33, or $0.129 per ounce. Costco is still the store to beat, but you have to buy a 70 oz. box. For 70 ounces, the Wal-Mart price will be $9.06, compared to $8.49 at Costco. So, I can save $13.71 a year if I can finish two 70 oz. boxes a month.