This is correct. Liliana, Heretical Healer's Mana value is 3, since it's the back face of a transforming double faced card.Creatures that Flip into Planewalkers don't change their mana value to 0.
example: Casting Culling ritual won't destroy planeswalker form of Liliana, Heretical Healer.
They changed it? Oh ok I didn't know that. Thanks. I was looking at this rule from gatherer:This is correct. Liliana, Heretical Healer's Mana value is 3, since it's the back face of a transforming double faced card.
202.3b: The mana value of a transforming 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 transforming double-faced card (even if the card representing that copy is itself a double-faced card), the mana value of the copy is 0.