Stardock's sci-fi 4X Galactic Civilizations II might be a little long in the tooth (especially as GalCiv III is a thing), but even 11 years on from its release, it remains a solid offering in the genre. Still, I can understand why you might pass on it at $US19.99 ($26)... but what about zero dollars?
This rather competitive price, currently available at the Humble Store, gets you the Ultimate Edition. This comes with the game's two expansions, Dark Avatar and Twilight of the Arnor.
Dark Avatar, released in 2007, brought new units, races and a fresh campaign, as well as asteroid belts and "enhanced AI". 2008's Twilight of the Arnor had its share of features and content too, such as unique tech trees, and included a spruced-up graphics engine.
At the time of writing, Galactic Civilizations II will be free via the Humble Store for another 40 hours. Note that it comes in the form of a Steam key, rather than a download from Humble.