The price for the bundle is set by the customer. So you can pay as much or little as you want for it. Proceeds go to the game developers, charities, and Humble Bundle. The game set includes Amnesia: The Dark Descent, LIMBO, Psychonauts, and Superbrothers: Sword & Sworcery EP. Bastion is unlocked only if you pay more than the average price for the bundle.
For more information see the announcement on Canonical's blog.