Wretched Gryff

The mana value of the back face of a double-faced card is the mana value of its front face. The mana value of a melded permanent is the sum of the mana values of its front faces. A creature that’s a copy of either has a mana value of 0.

You may sacrifice a creature with a mana value of 0, such as a token creature that’s not a copy of another permanent, to cast a spell for its emerge cost. You’ll just pay the full emerge cost with no reduction.

The creature chosen to be sacrificed is still on the battlefield as the cost of the emerge spell is determined and as you activate mana abilities to cast the emerge spell. Its abilities may affect the spell’s cost, be activated to generate mana, and so on. However, if it has an ability that triggers when a spell is cast, it will have been sacrificed before that ability can trigger.