You can grab (two copies of) War of the Roses: Kingmaker, March of the Eagles, Darkest Hour: A Hearts of Iron Game and Sword of the Stars 2: Enhanced Edition for whatever price you wish to name.
Hit the average price, currently $2.51, and you'll also net yourself Cities in Motion 2 and Warlock: Master of the Arcane. Pay $16 or more and you'll unlock the peerless Crusader Kings 2 and its Russian and Norse Unit DLC. That should keep you going roughly forever.
All of these titles are redeemed through Steam, and only Cities in Motion 2 and Crusader Kings 2 are available for Mac. As usual you can split your money between Humble, developer Paradox and this bundle's chosen charity - charity: water.