Green Man Gaming offers downloads of the Civilization Franchise Pack for Windows for $25.49. Coupon code "GMG23O-FFLABO-RDAYUS" cuts it to $19.63. That's $14 under the lowest price we could find for these games sold separately elsewhere. This pack includes Complete editions of Civilization III, Civilization IV, and Civilization V.
Note that a Steam account is required to download and play this game.