Sometimes, but like the base game, the discounts aren't huge. Maybe 20% max.

The newest three are currently 12-14% off at fanatical.

https://www.fanatical.com/en/search?search=rimworld

I will say that biotech is probably my favorite DLC, but if you don’t want to buy it now there are mods that add androids

Ditto

They do go on sale (but not for much less), but if you're not strapped for cash then the base game as well as the DLC's are well worth the money.

To answer your question, yes, the game and DLCs go on sale periodically, nothing super crazy, usually topping out at about $5 off or so.

In the spirit of teaching a person to fish and all, you may find IsThereAnyDeal useful. It's got price histories for most platforms and key vendors* and if shopping sales and bundles is your thing you can set up email alerts.

* personally, I like Fanatical or Humble, (and Steam) since their keys are all nice and official, but if you're grayer markets are your thing those are there too

Thanks!!