When it comes to ingredients, buy the best you can afford. But keep in mind that the best is not necessarily the most expensive.
Vegetables
I always like to start a trip to the supermarket with fruits and vegetables. I pick out what's fresh, and plan my meal around it. In the warmer months, I like to go to the farmers' market, and in the winter, I eat a lot of root vegetables. I always keep frozen peas and spinach and canned tomatoes on hand. In fact, I use canned tomatoes exclusively in the winter because it just isn't worth paying $3 a pound for grainy bland "fresh" ones. I try to buy organics of the stuff that is most likely to contain pesticide residues, even after washing.
I have a veggie bag in my freezer where I keep unusable ends/skins of veggies for when I make stock.
Meat
It's worth buying antibiotic free meat.
The paradox of meat is that you want a nice, carmelized outside to seal in the flavor, and this requires high heat. But you want to slowly heat the inside to just-doneness or else the meat gets tough. If you start out by searing on high heat while turning every 1-2 minutes, and then finish on low heat in braising liquid, you can make any meat tender. Marinades can also tenderize tough pieces of meat. This is why, if I'm making anything where the meat ends up cut into small pieces, I always buy the cheapest stuff, which is usually the "stew meat." This also saves labor, as they've already cut it up for you!
I always keep any bones for stock. You can keep the bones in the freezer until you're ready to make your stock.
And I keep my bacon grease. There are loads of ways to use smoked lard.
Chicken
I buy whole chickens. I like to roast them, then use the leftover meat for chicken salad, then use the scraps and bones for chicken soup. One chicken is enough meat for two people for a week. Seriously.
Fish
Trout, when you can find it, is cheap, delicious, and often local. And since it's a vegetarian fresh-water fish, it's very low in mercury. I also like to buy individually-frozen tilapia loins from Costco...they make an easy weekday dinner and are farmed in fresh water, so they're among the more "sustainable" fish. But because I live 1000 miles from the nearest ocean, seafood is a luxury.
Beans
It's cheaper to buy dry ones instead of canned, and you can make them much more flavorful than canned ones by cooking them in stock. I like to add bacon or ham hock to mine to make them a bit more substantial, and so delicious I don't miss meat.
When I do buy canned beans, I try to find those with no added salt.
Cheese/Dairy
It *is* worth spending the extra money for dairy products without recombinant bovine growth hormone. Your body will thank you for it.
Fats
I keep olive oil, grape seed oil, butter, and bacon grease on hand. If you can't afford grape seed, canola is an okay substitute. Olive oil has a low smoke-point and is thus unsuited to cooking at high heat.
Unless you're baking, you can substitute olive oil for up to half the butter in a recipe.
I always use unsalted butter. If using salted, adjust the salt in the recipe accordingly.
Spices and Herbs
I buy all my spices in bulk and replace them annually. I spend only about $30-$40 annually on spices and dried herbs, mainly because I'm not constantly buying extra tiny glass bottles for them.
I only use fresh onions and garlic, but I do occasionally use powdered ginger.
I use dried herbs in the winter, but grow a few fresh ones (thyme, oregano, basil, mint, and sage) in the summer. In general, you can substitute any dry herb for any fresh one by halving the amount.
Parsley and cilantro are cheap year-round, so I always use fresh.
Stock
It's so worth it to make your own. You can save up bones and ends of veggies in the freezer, and make the stock once you have enough. If you're short on freezer space, you can reduce the stock and then freeze it in ice cube trays. This is much cheaper and yummier than buying stock at the store, and it's easy...seriously, two minutes of prep and then you can go watch a movie. Vegetable stock should include carrots and mushrooms if substituting for beef or veal stock.
