There is a major Call of Duty sale on steam right now.
From now until September 21st, almost every game in the series is available at a significantly discounted price. Also included are various bundles, one featuring every game and DLC, excluding Advanced Warfare, for $200. While pricey, the final cost without the sale is well over twice as much.
Individually, most of the games are around 10 dollars, except for the three most recent entries which are 20 or 30 dollars.
A complete list of prices and bundles can be found here.