Picking up on the 'Costco' element...
...they've recently opened here in Canberra. Yep, out we trudged to become members and see what bargains we could get. Well, some of their items are cheaper, especially if you are a big brand buyer. However, if you tend to buy smaller brand names or 'own' labels then the savings are not that great .
The other kicker is that many items come in 'economy' sized or multibuy packaging. When there are only two of you in the house this does cut down somewhat on the savings particularly if you end up throwing the last small amount away....
Aldi and supermarket specials, on-line retailers and discount retailers tend to offer an overall better range and at the minimum, comparable savings.