Awesome pack of XCOM games including Enemy Unknown + DLC, on Amazon US.
XCOM: Enemy Unknown
XCOM: Enemy Unknown: Slingshot Pack
XCOM: Enemy Unknown: Elite Soldier Pack
X-COM: Terror from the Deep
X-COM: UFO Defense
Note 1:DOES NOT include: The Bureau
Note 2: AFAIK All Steam, not sure if split or single keys
Note 3: Price is at current exchange rate, DOES NOT include charges/fees applied by your bank/paypal/card provider/etc.
Note 4: Amazon US, so usual US Amazon address trick required.
If you want a XCOM:EU/The Bureau pack, check this post:
EDIT 1:Price has been lowered further to $9.99 (£6.19), even more of a bargain now! Thats cheaper than buying UE on its own without the DLC! They may change it so be fast!
-Thanks to Whizzey for the price update! ;)