If you look in today's edition of The Wall Street Journal, you will find this amusing "puzzle" to share with your students. The set-up, Marshall and other 19th century economists noted that producer cost was the sole basis of price. Consumer value had to play a part, as well. Essentially the consumer's willingness to pay had to relate to the utility they received.
What utility is received from a vanity plate? I was taught that there were three kinds of utility - form, place and time. I have my idea, but I'd be interested in your thoughts.