You can make your own dip for potato chips, if you are allowed to eat it. Buy dried onion soup and mix it with sour cream or yogurt. It may not be less expensive, but it sure tastes better than what you buy in the dairy section.
Make your own nonfat (or low-fat) yogurt. It is better for you, it may not be cheaper. To a carton of nonfat yogurt add frozen juice concentrate. You may need a little bit of a sweetener. My family likes the combination of apple juice and cinnamon, mm. Experiment a little and you may find a snack good for your family, too.