Beans of all types (dried will be the cheapest) but even canned beans are a steal.
I usually buy the largest bag of black beans I can find. Cook them add salsa and cumin to flavor and then freeze in small batches (the gladware works perfectly for this). Then when I need a meal that can use a fiesta black bean touch I just thaw and heat. (bean burritos) (stir-fries).
Another economical item is TVP (Textured Vegetable Protein). You can buy it in health food stores and dollar per pound it's cheaper than any meat you can buy. It's great protein and an awesome addition to add to stews/soups to make them more filling and last longer. It never goes bad as long as its stored in an air tight container. Buy it in bulk and use as needed.
Pasta as others have said is something that can be bought in bulk and used on the cheap. I prefer whole wheat since the nutritional content is way higher.
Finally a cost effective way to save money is to do the following:
What you need:
5 x 7 cards with holes punched
5 x 7 card binder
a pen
a calculator
conversion table if needed - ie. oz to lb, etc.
Put the cards in the binder
Bring it to the store
When you buy something put the name of the item on the top of one card
Then underneath, put the:
date, store, price, units, price/unit (ie. this is where you may need the calculator)
Then keep doing it. You use index cards in a 3 ring binder because then you can shuffle them to alphabetical order before you hit the stores the next time.
To get you rolling, the first trips to the store, go thru the cards you have items for and write down the prices/units of all the items you have cards for already, especially if you are at a different store. You may also want to write down a few items the first time that you know for a fact you are going to buy later...
toilet paper
paper towels
bananas
peanut butter
etc.
not too many so you feel overwhelmed, but just a few, and price them on your next visit.
After you have done this for a while, you'll find some surprising things... I found that Tide laundry detergent (something I really like, and don't bend on), is cheaper at Ralphs grocery store, than Walmart, Costco, etc.
Saline solution for contacts is 70% cheaper at Costco.
Or at least these were true last I used the book, I need to get back on it.
If you do it diligently enough, you'll even be able to correlate times of the month or holiday weeks when certain things are cheapest.
It all goes to price/unit.
After a while, it'll be ingrained in your head which store to pick up what without looking, or actually if something is on sale, the numbers will be burned into your head whether it's a good deal and you can stock up. Once you start buying some of these things in bulk during good deals, you'll start seeing in your budget spreadsheet that your expenditures will drop... during the bulk building stage though, some numbers will increase of course.
