Not everything is cost efficient to buy in bulk - mostly, when you buy in bulk and don't even use all the crap you bought. I try to know what is best to buy in bulk for our family.
Here's my example: Garlic Stuffed Olives. I love them. This huge jar is only $6.99 at BJ's. At the grocery store, a jar about 1/3 this size sells for $5.99. I eat them like they're candy so it's worth it for me to buy bulk. My husband thinks I'm nasty.