Most likely a price mistake, so I would act fast, but Age of Empires III is 10 cents/10 Microsoft Points on Games for Window Live. Only shows up in the client its self though.
Edit: Nope, not a price mistake, just a sale that went up early. Can also be gotten on the GFWL website now:
http://www.gamesforwindows.com/en-US/Games/AgeofEmpiresIII/
Edit: Nope, not a price mistake, just a sale that went up early. Can also be gotten on the GFWL website now:
http://www.gamesforwindows.com/en-US/Games/AgeofEmpiresIII/