The phrase "different mana values" compares the mana costs of cards in your graveyard to one another, not to Seasons Past. You may return a card with a mana value of 6.

If a card in your graveyard has {X} in its mana cost, X is considered to be 0.

A card with no mana cost, such as a land, has a mana value of 0.

You choose which cards to return as Seasons Past resolves, not as you cast it.