By and large, the less a food has been touched by human hands, the cheaper (and frequently healthier for you) it is. There are a few exceptions, like frozen veggies, but it’s a good rule of thumb. Whole raw chicken is cheaper than cut-up whole chicken, which is cheaper than skinless, boneless breasts, and all of those options are cheaper and better for you than an equivalent weight of microwavable chicken nuggets. Loose corn in the full shuck is cheaper than those packs of partially cleaned ears, that sort of thing.
Build up a stable of fairly easy, inexpensive recipes that have interchangeable ingredients or that you can roll one night’s leftovers into the next day’s meals. This will take a bit of time, but it’s not too hard once you get used to thinking about meals in terms of their common ingredients. It means you’ll have to plan your meals in advance, which takes a little getting used to but is well worth it.
One of my personal favorites in this vein is a whole roasted chicken. Roast the chicken one night, and serve it with…well, whatever you feel like, really. The next day, pull of some of the meat and put it in a pita with some chopped onion and shredded cheese and nuke it. There’s your lunch. For dinner you can make a big salad and put some of the meat on there with a bit of cheese and whatever fruit you have handy, maybe a few nuts. Tomorrow you can have quesadillas, or pour a little barbecue sauce on, or…you get the picture. If it’s just you, you’ll probably get tired of the chicken before you run out of meat or ideas.
Invest in freezer bags or containers. Most recipes make enough food for four or so people, so instead of cutting it down, make the full amount, eat one portion and freeze three. This is especially nice for nights when you drag in late, worn out and starving–instead of having to cook, or grab a burger someplace, you can just pull a container out of the freezer and nuke it.
Learn to love your slow cooker. If you don’t have a slow cooker, either buy one or put it on your Christmas list. You can make damn near anything in one with very little effort, and there are tons of healthy recipes out there. Even recipes that aren’t meant for the cooker can be adapted, especially soups, stews, bean dishes, and roasted meats. If it’s meant to be cooked low and slow, the crockpot is the way to go.