Paradox is supposed to have a policy of not offering products with discounts of more than 50 percent before they are more than six months old.
In fact, Wealth of Nations was released at the end of May, so it's only just over five months old. This was a pleasant surprise for me as I have a policy of not buying DLC with a discount of less than 75 percent. Res Publica was released in July, so I'll probably have to wait for a Christmas sale at least for that.
I might relax things a bit for Art of War and wait for 50 percent.
The Native American and Conquistador unit packs have a 75-percent discount at GreenMan Gaming and Humble.
Of course, this sale isn't so much about Halloween as about the release of Art of War.