The total purse is actually $30 million, with $20 million for the grand prize and $5 million for second place and $5 million for performance bonuses. (Second place would really be a bummer.) To win, the robot must move for at least 500 meters and be able to send video, images and data back to Earth. The grand prize will decrease to $15 million after Dec. 31, 2012, and it will remain at that level for another two years. After that, organizers will decide to extend or terminate the project.
The X Prize Foundation is no stranger to lofty goals, of course, having offered a few multi-million dollar rewards for science and exploration over the years. Most notably in the aerospace realm, the $10 million Ansari X Prize was awarded in 2004 to Mojave Aerospace Ventures (Burt Rutan, Paul Allen and co.) when they successfully (technically) sent a human into space twice within two weeks aboard SpaceShipOne.
NASA has said it wants to send astronauts to the moon by 2020. When you remove the human element from the Google Lunar X Prize it's certainly a safer -- and SLIGHTLY easier -- proposition. That said, it will still be extremely expensive and likely not profitable for any private company or entrepreneur involved. In fact, you could say the budget will be out of this world. (Though for Google it may mean planting its flag.) But even with all that cost, challenge and potential heartache there are those who will still try. And isn't that the spirit of space exploration?