Bastion is being offer free via IAP. Previous version was published by Warner Brothers and Supergiant Games are now switching to self publishing.
This coincides with their 10th anniversary celebration, so the release of the self-published Bastion is free to download, with free IAP to unlock the full game.
Note you must download the game, then go into settings to purchase the IAP (should show $0.00).
See their blog post for further details: