wotc
If a card in your graveyard has {X} in its mana cost, X is considered to be 0.
If a card in your graveyard has {X} in its mana cost, X is considered to be 0.
You choose which cards to return as Seasons Past resolves, not as you cast it.
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.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Christine Choi
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
A card with no mana cost, such as a land, has a mana value of 0.