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The cheapest this collexion has ever been is $20, so this is a great deal. It expires about 3 PM East Coast time, so make haste.
While it is called the Complete Collexion, it doesn't actually include the DLCs released this year, only the first 21. If you are a patient type, you could wait for a true Gold edition to be released one day.
Great price for a great game, still $79.99USD on Steam
Historical Low: $11.99 at Humble Store on Thu May 08 2014 - According to Enhanced Steam Browser Plugin (which I highly recommend)