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This is the lowest price I have seen on the Waterborne DLC and ties the lowest price for Tropico 5 as well. All of the other DLC is 50% off as well and they all activate on Steam.

Tropico 5 Waterborne [macgamestore.com] $ 9.99
Tropico 5 [macgamestore.com] $ 9.99
Other Tropico 5 DLC [macgamestore.com] Most are $ 1.99