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.