Beans come in dried and canned variety. Dried are a lot cheaper, but the prep work is intensive - soaking and then cooking takes 45 minutes to 90 minutes. That's a lot of time, and it's a lot of fuel. If you've got say a woodstove and plenty of wood, not such a big deal. If you're cooking off of a camp stove with a limited supply of propane, it becomes a bigger deal quickly.
Canned beans are a bit pricier, but they are really convenient and18 make using up your beans much easier. Open can, add to dish. We add them to soups, chilis, serve them as a side dish and mash 'em up for refried beans.
We store black and pinto beans. I generally like varieties that come with some kind of seasoning out of the can - they're usually pretty tastey and don't require much extra work to make 'em into a good dish. I wouldn't be afraid to mix up your bean variety - throw in some baked beans, chilis, etc.
Cans cost about .69 to $1.29 or so. Look out for sales and buy cheap brands. If they do have seasoning, I'd try out a can of two before clearing the shelves.
Beans are loaded with protein and fiber, and pretty dense calorically, too. One 10-oz can of black beans, as an example, will have around 420 calories, 18 grams of fiber, 24 grams of protein.
Since we're gunning for a month worth of food for a single person, go ahead and pick up around 30 cans of the beans of your choice. So, depening on the brand and price you'll pay, you're looking at $20 to $30.
Rice and beans have zippo in the way of fats, so you'll want to add some when you cook 'em up. Olive oil is what is usually called for in recipes, and most people already have some in their pantry. If you're low on it, pick up a bottle to keep in reserve. Other oils will do in a pinch as well. Oils need to be rotated around once a year, so it doesn't make sense for most people to stock up a huge quantity.
If you've got extra money left in your $40, go ahead and spend it on some additional canned goods/shelf stable food, to help add some variety and supplement the standy-by rice n' beans. Ramen/cup-o-noodles might not be a great health choice, but they're cheap and they're food. Some bulk quick oats or bisquik can give you a change of pace for breakfasts. Cans of meat, soups, stews, peppers and tomatoes can add some variety, too. If your spice cupboard is lacking, maybe make a few additions - salt, pepper, bullion cubes, bulk spices and sugars are cheap and one can do a lot with 'em.
Our case of rice and a can of beans a day will stretch us over that month that we're looking for - it's approximately 66,000 calories, or a healthy 33 days worth of 2000 calories a day. Add that onto whatever food you already have at home, plus the supplemental food you've bought this week and last, plus the 3 days of food in your bug out bag, and you're sitting fairly good. In the CONUS, I don't know of anyone in recent history who has been unable to access outisde food for over a month, so it is a pretty good baseline.
Next week, we'll finish off the low-budget one month food supply and move into the last portion, barter goods! Stay tuned. It's been a fun ride and we're almost done.