XCOM: Enemy Unknown is currently $10 through Amazon's digital distribution network. We don't believe this is a Sectoid conspiracy, but that's a pretty good deal, especially since it includes both the Elite Soldier and Slingshot downloadable content packs.
XCOM's Enemy Within expansion, which will add new tech, enemies and maps, is currently scheduled for November 12. The expansion is priced at $30 on PC, so if you've yet to pick up XCOM, this deal would be the same as getting the Xbox 360 or PS3 "Commander Edition." That version includes a copy of Enemy Unknown, along with the Slingshot and Elite Soldier content packs for $40.