Though the game has been out since 2010, Civilization V is still selling strong through the years. If you're a turn-based strategy fan and you somehow, magically, inexplicably don't have it in your library, consider the Complete Edition now that it's $10 this week in Green Man Gaming's Civilization franchise sale.
In this sale, Civ V, IV, and III are all 75% off, plus there's a stacking 20% code you can use too. All the keys are Steam Play titles, meaning they'll also work on Mac without an issue.
If you have Civ V already and you just want the DLCs piecemeal, they're discounted as well.