In their Facebook page, Eagle Dynamics describes the full details of the promotion:
Starting September 5th, 2018 and lasting until September 30th, 2018, we are offering all four DCS World helicopters for just $69.99! Normally a $199.96 value; this is a 65% saving on each helicopter. DCS World offers some of the most realistic helicopter simulations available on the PC, and nothing compares to the helicopter combat environment that DCS World offers.
For new DCS World helicopter players, this is a rare opportunity not to miss. Find here on September 5th, 2018:
DCS: UH-1H Huey
DCS: Mi-8MTV2 Magnificent Eight
DCS: Ka-50 Black Shark
DCS: SA342 Gazelle