As a graduate student, I don't make very much money. I try to get some of my pantry staples at Costco, where I can buy in bulk and save. These are the usual items I purchase there, although some items last a long time and I don't need to buy them every month. The best part is that lots of their items are organic, like frozen fruits, frozen veggies, and PB&J.
Perishable items: bananas, baby carrots, fresh fruit on occasion, bread
Non-perishable items: milled flax seed, quinoa, peanut butter, strawberry jelly, roasted red peppers, sun-dried tomatoes (oil-packed), Clif bars, pretzels on occasion, Basmati rice, Kalamata olives, vanilla soymilk
Frozen items: strawberries, blueberries, broccoli, mixed vegetables, sweet potato fries, and rarely Boca burgers (used to buy these a lot in college, but I don't now)
Household items: detergents for dishes and laundry, shampoo & conditioner -- THE STORE BRAND IS USUALLY CRUELTY-FREE! Kirkland Signature dish and laundry detergents are advertised as not tested on animals and the moisture rich shampoo & conditioner say 100% vegan on the front of the bottle. I personally think it's awesome that Costco is willing to make eco-friendly and cruelty-free products for the store brand!!
The savings more than make up for the membership cost. For example, I eat a Clif bar every morning before going to the gym. The box of 24 bars is $18. At the regular grocery or Whole Foods, Clif bars are $1 each or a box of 6 for $5.50 (or up to $5.95 at some places). I basically save $6 for each Costco box of Clif bars. I save *at least* $72/year on this single item, which covers most of the $100 membership cost. :)