Why is a moon rock worth $10 million?

Honolulu Advertiser today (January 11, 2010) had a story about some moon rocks that turned up during an inventory of the governor's office collection of gifts. What is interesting is that the rocks were called "priceless", and valued at around $10 million. They have a picture of one, it looks like a piece of lava from the Big Island, and is encased in resin.

Now, why exactly is it priceless? What is the $10 million value based on? If it is the cost of the lunar missions divided by the number of rocks brought back from the moon then I can understand how they calculated the estimated value. Otherwise, the rock itself has actually no value, it is just a rock like any other rock, it just cost a lot of money to go and get it and bring back!