Bob went to the hardware store to make a purchase for his house.
He asked the store clerk, "How much will one cost?" The clerk thought for a moment and said, "Three dollars."
Bob looked a little puzzled and said, "Well then, how much will twelve cost?" "Six dollars," replied the clerk.
Bob scratched his head and said, "If I were to purchase two hundred, how much would that cost?" "That," said the clerk, "will cost you nine dollars."
What was Bob buying?