Steam is implementing a change to the way bundle prices work, replacing the flat cost with a dynamic pricing model instead; that decreases the price if the user already owns one or more of the products in the bundle.
The way the pricing system will work depends on how many of the featured games a user has in their library, with the discount increasing for the each additional game the user already owns.
“With Steam Bundles, if a customer already [owns] some items in the Bundle, they will pay for and receive only the items not already in their account,” the documentation reads. “This allows the best fans of your series or franchise to ‘complete the set’ and get a deal on the remaining items in the Bundle.”
Valve believes that bundles and complete packs are “a bad deal” for customers that already own a large portion of the included games.
“Either it made bad economic sense for those customers to purchase the pack, or they just felt bad about doing so since it [looked] like they were paying for products they already [had]. The new Steam Bundles system addresses this.”
This will only apply to bundles purchased directly through Steam, and not extend to third party bundle sites like Humble Bundle.