Chiming in with my two cents; I will never pay fifty bucks for an expansion, even if the base game comes ‘au gratis’, when I have no need for, and will never HAVE a need for, a second CD-key for the base game.
Fortunately, I am blessed with sufficient patience that I can wait a year or more for the appropriate measures to be taken, either a separate upgrade price, without the ‘free’ anchor, or a sale which drops the price by at least half, to the appropriate price range for expansions.