Really for it to be profitable you have to know how much you pay for your normal stuff.
As a recent example, my wife and I were completely out of chicken broth (yes, I know we should make our own, but we don't, so deal with it!) -- something we try to keep on hand because we use a fair bit of it. The Swanson low-sodium kind at Target is $2.99 per "box", a case of 12 cans at Costco was like $9 or something. Well, one "box" is equal to 3 cans .... 12 cans / 3 cans per box = 4 boxes * $2.99 = $11.69. Costco savings = $2.96.
Their meat section is pretty good also, 88% lean ground beef is $3.19/lb. Not quite as cheap as a cash-only market we go to sometimes, but close enough where it's not worth driving to. $4.99 for a rotisserie chicken isn't bad, we got 3 meals for 2 people out of a free one we got for a promotion. Milk is $.30-$.40 cheaper per gallon than anywhere around here. Shredded cheese is awesomely cheap, it works out to ~$1.29 per small bag that is normally $2.49 on sale at the grocery stores. We also found their vitamins / supplements to be VERY well priced. Huge bottles for the same cost as about half the size at Target/Walmart.
They do have monthly "ads" available at the front of the store, usually at the customer service / signup desk. Some of it is very well priced (5 big tubes of Crest toothpaste for $8 last month, plus applied toward the $33 back on $100 Proctor & Gamble rebate...).
TLDR: Just gotta know how much you normally pay for stuff.
