Whiffle balls seem horribly inefficient in terms of the amount of space they occupy compared to the amount of bacteria that can occupy them. As already mentioned, sanding them will help - also, cutting them up into quarters would too (so that they don't waste so much space). For about the same price as what you're thinking of spending however, you can get 4kgs of SeaChem matrix, which will be far superior in terms of bacteria real estate.
Then there's the question of plastic toxicity - I doubt whiffle balls are made to be aquarium safe.
Then there's the question of plastic toxicity - I doubt whiffle balls are made to be aquarium safe.
