En las reglas actualizadas explica que es el coste impreso en la parte de adelante de la carta. Tal vez encontraste alguna referencia vieja, de cuando funcionaba diferente.

http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules

202.3a The converted mana cost of an object with no mana cost is 0, unless that object is the back face of a double-faced permanent or is a melded permanent.

202.3b The converted mana cost of a double-faced permanent’s back face is calculated as though it had the mana cost of its front face. This is a change from previous rules. If a permanent is a copy of the back face of a double-faced card (even if the card representing that copy is itself a double-faced card), the converted mana cost of that permanent is 0.
Example: Huntmaster of the Fells is a double-faced card with mana cost {2}{R}{G}. Its converted mana cost is 4. After it transforms to its other face (Ravager of the Fells), its converted mana cost remains 4.
Example: A Clone enters the battlefield as a copy of Ravager of the Fells. Its converted mana cost is 0.
Example: Insectile Aberration is the back face of a double-faced card whose front face has mana cost {U}. It becomes a copy of Ravager of the Fells. Its converted mana cost becomes 0.


711.4b While a double-faced permanent’s back face is up, it has only the characteristics of its back face. However, its converted mana cost is calculated using the mana cost of its front face. This is a change from previous rules. If a permanent is copying the back face of a double-faced card (even if the card representing that copy is itself a double-faced card), the converted mana cost of that permanent is 0.