Ground beef: I like to buy my ground beef in the large five pound packages, which I take home and promptly divide up into six "one-pound" (-ish) portions to put in my freezer. I don't notice that each pound is slightly shy, and I get an extra meal out of each five pound package I buy.
Couscous: Couscous is one of those ingredients where the prices vary, a lot! I've gotten it in the bulk aisle for pennies a person, and I've seen the same plain couscous in a small box in the gourmet aisle costing five or six times as much. So I load up in the bulk aisle or when the boxed or bagged couscous goes on sale. I keep my stock of couscous in canisters in my pantry so that I'm not at the mercy of the gourmet aisle prices.
Cumin, cinnamon and other spices: Some grocery stores have spices available in bulk (usually in the baking aisle); you pay by the ounce. I cannot believe how inexpensive spices are when you only buy a little! So if your store offers that option, be sure to take the few extra minutes to see what's there – those five minutes could save you big. Often, however, spices have to be bought in regular spice jars, so I tried to take some of the flavors from North Africa that we use more often in the US to leverage your pantry as much as possible.
Cilantro: Fresh cilantro is incredibly inexpensive when it's in season. I've gotten bunches for 25 or 30 cents each. When they are cheap, I buy seven or eight bunches and blend them up with a little olive oil. I pour the mixture into ice cube trays (fill halfway) and freeze, and then keep the cubes in a resealable freezer bag. The cilantro will taste (almost) as good as fresh for months.