Civilization V, the most recent game in the Civilization series (unless we count the new Facebook game), can be yours on Steam for just $17. Even at $50, the amount of time you could spend playing it makes it way more than worth the price of admission. For less than half of that, strategy fans would be crazy not to pick it up.
If you’re looking for something cheaper or less combat-centric, Dawn of Discovery Gold (known as Anno 1404 outside North America) is $6.79 from Impulse, which is also selling Dead Space 2 for $17.99.
Steam has the very fun Altitude for only $4.99. You can play the full game for free this weekend to first get a taste to see if it’s up your alley.