Rule 202.3b

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