Originally, Rheia's Gift costs 5 favor. With patch 1.02, it costs no favor.
Rheia's Gift reduces favor cost by 25%.
Tale of the Dragon
With patch 2.7, Rheia's Gift reduces favor cost by 50%.
As the children of Kronos and Rheia were born, Kronos swallowed them one by one. When Rheia gave birth to Zeus, she hid the infant and fed Kronos a stone instead, risking her husband's wrath to save her youngest son. Rheia is held in high regard by both the Olympians and the Titans, and her son Zeus - her gift to the world - is honored by all.