Cheap popsicles. Relatively low calories (<100 depending on the brand), take a while to eat, and are extremely cheap.
If you make your own you can drive the price and Calories even lower and drive the flavor way up.
Side note: I just learned "popsicle" is the brand name and not a term for the food. Kind of like saying Kleenex instead of tissue. I had no idea, I've just always called them popsicles.