RimWorld: Royalty Review (Builder)
Buy this one last. Adds a fair bit to the far end of the tech tree, a new faction that's too strong and drops too little loot to be worth raiding (and can't be positively interacted with without the nobility system), a nobility system that itself doesn't necessarily improve your colony, and psychic powers that are honestly not the most balanced. Oh, and all the quest additions are actually pretty significant, because the base game's get pretty repetitive, and the other two DLCs don't add many. Greatly improved by mods, and several mods rely on it.
Doesn't add any true core mechanics like Ideology's... ideology, or Biotech's children and races. Those two are true expansions with a focus on one specific aspect of the game, but this is closer to a content pack. Still worth the money, just, the least so out of any DLC for this game. And hey, The Sims is asking $40 for basic features like pets nowadays. Compared to that, you're robbing Tynan blind.
Keep in mind, DLC for this game only goes on sale for 10% off. As Royalty gets older that might bump to 20%, which is the most the base game goes on sale for. But you probably shouldn't bother waiting for a sale unless you're really stretching your budget.