These are the simplest of healthy snacks, those that even complete non-cooks should be able to manage.
Popcorn is a favorite, simple and low-cal, and very cheap...if you avoid the microwave type. If you get a hot air popcorn maker (which seem to be making a bit of a comeback), you can buy popcorn in large bags for less than a box of the gourmet microwave popcorns. And you can completely control how much butter and salt you use. I say butter intentionally because we've found that margarine tends to make the popcorn turn soggy while butter does not. And if you want fat free, keep in mind that all microwave popcorn adds some fat, while none is required for hot air popping.
Crisp cold raw vegetables. Carrots, cucumbers, celery, cauliflower, mushrooms, brocolli, zucchini, radishes, onions, green peppers, and ripe (red, orange, or yellow) peppers all are quite good served raw. Cherry tomato or low-juice tomato slices are also good, but have more potential to be messy. Serve with a simple dip---I like to mix 2 parts plain yogurt and 1 part olive-oil based mayo with a bit of seasoning (things like onions, chives, parsley, dill, salt, pepper). Or use salsa. Carrots and celery are quite good with a little peanut butter. Pick vegetables that are in season and don't overdo the dip, and this is not only nutritious but cheap and filling.
Fresh fruit. Grapes, berries, sliced apple, pear, pineapple, and bananas can be eaten with a sweet dip...I like plain yogurt with some honey stirred in. Again, pick things in season and go light on the dip for nutritious, cheap, and filling.
Berries and milk. This is one of my favorite desserts, something I only get to enjoy for a few months of the year. Wash the berries. Cap them if they're strawberries, and slice large strawberries into quarters. Put the berries in a bowl, pour on enough milk to cover. Crush and add some unsalted nuts and toss on a few chocolate chips. Sprinkle a little sugar on top if the berries aren't very sweet. A cup of berries is about 45 calories, half a cup of skim milk is about 45, a 1/4 cup of nuts is about 50 calories, and add a few chocolate chips and some sugar, and you have a great dessert for between 175 and 200 calories. You can mix several types of berries and nuts or have just one kind of berry and nut each. I leave off the sweeteners sometimes when berries are really cheap and have a bowl for breakfast with a slice of peanut butter toast.
Peanuts, sunflower seeds, and pumpkin seeds. These are generally pretty cheap and fairly nutritious. Peanuts are not a complete protein, I'm not sure about the other two, and if you're watching sodium, you probably should consider avoiding the salted varieties.
Dried fruits. These can be a bit expensive, other than raisins, and sometimes dried cranberries, but a great energy source if you like them.