For its members, Green Man Gaming offers preorders of downloads of Sid Meier's Civilization VI for Windows for $46.79. (You must be logged in to see this price. Not a member? It's free to join.) That's the lowest price we could find by $13.
Also available for members are preorders of downloads for the Deluxe Edition for Windows for $62.57. That's another price low, this time by $17. This edition includes the game, 25th Anniversary Digital Soundtrack, and access to four post-launch DLC packs. Both games are expected to be released on October 21.