At a cost of $40 for the membership, a 60 mile round trip at 20mpg and probably a Taco Cabana meal thrown in, there needs to be a bit of savings in there somewhere. (My guesstimate, at least $25 cash out of pocket per trip).
A note about the shopping experience... It is a different world once you go inside the giant warehouse store. Packaging and quantities are not the same that you find elsewhere.
One of these is not like the other...
Look closely at the quantity of formula in each package. The top one is from a multi-pack at Costco, which included that tub (17.5 oz) and two refill packs (Total 52.5 oz).
The bottom one (23.4 oz) is the standard single container from Walmart which goes for about $23.67.
Without looking carefully, the packaging looks like a double supply, but that would only be 46.8 oz.
Pricing comparison - Walmart, $1.01 per ounce. Costco, about $.96.
For Renee's favorite Kashi cereal, a two-pack seemed like a good deal, but we discovered when we got home that each package was 2.5 oz. smaller than a standard single box.
Of course, Costco also lures you in with monthly coupons, which for May included a whopping $8.50 off of the formula. So it is cheaper, but only significantly so with the coupon.
Brand-name diapers are a better savings.. we picked up a giant box of Huggies size 2 for 15 cents each, the cheapest I have found. I have also heard nice reviews of the Costco store-brand (Kirkland) diapers.
I don't know how Sam's compares, but I will just assume similarities... large quantities and some savings.
Bottom line.. if you live within a normal shopping distance or just like to stock up, you can save money. There are lots of other member benefits that they would tout as well.
However, it is also worth doing the math to see if you will save enough to make the trip. Just don't forget that unless you are a super-disciplined shopper, you will buy some big boxes of items you did not have on your list. And with different package sizing, you won't know exactly if you are saving money.