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.

“Five or more mana values among cards in your graveyard” means there are at least five different mana values among those cards.

Sanguine Spy's triggered ability checks the number of mana values among cards you control both when the ability triggers and when it resolves. If you no longer have five or more mana values among those cards when the ability resolves, you may not pay 2 life.

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.

The mana value of a land card is 0.