This one should be fairly easy.
A woman went to a store to make a purchase. She asked the clerk: "How much would 1 cost?"
"That would be 1 dollar," he replied.
"How much would 12 cost?"
"That would be 2 dollars."
"What about 128?"
"That would be 3 dollars."
What was she purchasing?
There are actually two (or maybe more) possible answers. The one I'm thinking of is a common item you probably see every day.
NOTE: The correct answer has been guessed. To see it, read the comments section.
Lateral Thinking Puzzlers by Paul Sloane has lots of classic riddles, clues and answers. Click here to order it.