That one cent difference is enough for me to declare Amazon the winner, in my situation.

- You didn't account for the 20% Amazon Mom savings on diapers/wipes/and formula. This makes those diapers $35.95, or $0.19 each.

- I still live in a tax-free Amazon state. You have to add tax to Costco price. Advantage: Amazon (~7% savings in MO, disregarding the ethics of not reporting your sales tax not withheld).

- Costco requires an annual membership. You should technically spread this cost out to reflect the true price of what you're buying. Assume $55/year unless you buy enough so that the 2% top-tier membership works in your favor. Amazon Prime is $79 for a family. We have the $39 student membership through my old .edu email. I know several families that split the $79 four ways. Advantage: It depends on which you commit to. I'd say it's a wash.

- Gas and time are not factored into this equation. Advantage: Amazon

- With Amazon Mom you can order extra and return up to a year later. So besides the "opportunity cost" of having a few hundred tied up in diapers/wipes, you should never run out of diapers (so waiting 2 days or a pack shouldn't be needed). We currently have 1,000 diapers in the basement ... we're ready for the zombie apocalypse.

The Amazon versus Costco debate can take up a whole thread, and people seem to be entrenched much like iOS vs Android. I'm on the Amazon side. We've seen tremendous savings over the past year, especially when we've also loaded our amazon account with gift certificates at 3-15% off face value.