Rules
If the mana cost of a card in your graveyard includes {X}, X is 0 for the purpose of determining its mana value.
Spells that were cast before Eris entered the battlefield count. If Eris was the first spell you cast this turn, the next spell you cast this turn is your second spell.
The mana value of a double-faced card in your graveyard is always the mana value of the front face.
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.