If it's dense strategy you're after, this Humble Stardock Strategy Bundle is well worth a look. For a solitary dollar ($1.50 in Australia) you can get fantasy 4X Sorcerer King: Rivals, which is worth it by itself — but you'll also get Dead Man's Draw, which is a strategy game with pirates. Quite the rarity.
But if you're keen to pay more than the average you'll get an avalanche of Galactic Civilizations 3 stuff: in addition to the base game three expansions are bundled in, including the Crusade, Retribution and Mega Events DLCs. Back in 2015, Richard Cobbett wrote that it's "still hands down the best modern way to conquer unknown space". At the time of writing that costs $10.14 ($16.43 in Australia).
Finally, if you've got $13 bucks ($21 in Australia) to spare you can unlock every Star Control game (Star Control Origins, Star Control 1, 2 and 3, as well as another Galactic Civilizations 3 expansion (Intrigue) and Ashes of the Singularity: Escalation. Check out the bundle here.