Historical Rules · 22 April 2021
Rule 711.4c
Official source text from the 22 April 2021 Rules release
While a transforming double-faced permanent has its back face up, it has only the characteristics of its back face. However, its mana value is calculated using the mana cost of its front face. If a permanent is copying the back face of a transforming double-faced card (even if the card representing that copy is itself a double-faced card), the mana value of that permanent is 0.