Steam is having a weekend sale on 2K games, with prices up to 80% off.
Civilization 5 Complete Edition for $16.50
BioShock Infinite for $7.50
Even the upgrades and add-ons are on sale, such as Civilization 5's Gods & Kinds add-on is $7.50, and BioShock Infinite's Burial at Sea add-on is $7.50.
For a similar deal, GreenManGaming.com is currently offering a sale on some of the most popular PC games of all-time, including Half Life The Orange Box for $5 (includes all Half-Life Games), and Portal 2 for $5.