The mana value of a split card in the graveyard is the total mana value of both halves of that card. It does not have two mana values.

You may cast the spell immediately. You must pay all costs for a spell cast this way.

The mana value of a double-faced card in your graveyard is always the mana value of the front face.

X is 0 when determining the mana value of a card in your graveyard.

The mana value of a land card is 0.