I am normally not much of a juice person - but I really like tomato juice when I am dieting. A big glass of tomato juice and some nuts or a piece of low-fat mozzarella cheese makes a nice lunch/snack. (And I can get large cans for 99 cents at the dollar store, too.)
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Popcorn is a great inexpensive snack. You can cook it in a hot air popper and have a low calorie snack for very little money. The calories go on when you use oil, butter, or margarine on it.
An easy low fat snack is to cut up your favorite fruit(s) to be dipped into your favorite flavored fruit yogurt.
Grab and go 100 calorie snack packs are great to have on hand for when you are hungry between meals. You don't need to spend a lot of money on prepackaged ones though.