You can get a good deal on the price of diapers and save money buying diapers at Costco if you purchase enough diapers in bulk to see the savings.
If you go to Costco and just buy a few boxes of diapers then you really won't save much money because you have to buy the Costco Membership.
If you only buy the same amount of diapers at Costco as you would at any other store then you won't save much if any money.
When buying diapers at Costco it's best to buy 2 or 3 months worth of diapers to see a significant savings and get the best deal on diapers.
My sister stocked up 4 months ahead on diapers for her 2 year old when she got her Costco Membership.
She had the money and needed the diapers anyway so she stocked up and got a bunch of diapers to last 4 months at a time.
Then 4 months later she stocked up again for another 4 months and so on until her 2 year old child turned near 4 and potty trained.
Then she purchased her son's pull ups through Costco as well.
You should also do most of your other shopping such as for groceries and toiletries when buying diapers at Costco to get the most savings and to take advantage of the membership fee.