A man walked into a pet shop, pointed to a large dog in a kennel and said: "How much do you want for that big dog?"
"Fifty dollars," replied the clerk.
"And how much for that small fellow over there?" asked the customer.
"One hundred dollars," was the reply.
"And for that tiny one?"
"Two hundred dollars," said the clerk.
The customer looked puzzled. "How much," he asked, "will it cost me if I don't buy a dog at all?"
Source: J. Braude's Treasury of Wit and Humor, 1964.