For me nappies are NOT cheaper to buy from costco.
A big box of toddler sized nappies is 148 for $59.
Coles/Woolies have them 81 for $32 (so 162 for $64) and sometimes cheaper than that.
It works out marginally cheaper for costco but once you factor in petrol etc i dont think nappies are on the 'worth it to wait' list.
I also selected them in ym flybuys 'my 5' so get 10% off that price which means more nappies cheaper price.

I have just started using Wella S&C and saw they stock it at the sale price I see in supermarkets... so they have good deals and they are around more regularly.
Means you *could* go to costco to buy thse things if they arent on special at the shops.

For us, i dont know if we will see $60 worth of savings to make the membership worthwhile, but it's FUN! It's a fun family outing and we get to do harbourtown while were there.