Soul Tithe
Enchantment — Aura
Enchant nonland permanent
At the beginning of the upkeep of enchanted permanent's controller, that player sacrifices it unless they pay 
, where X is its mana value.
Dave Kendall
Return to Ravnica (RTR)
• #23Rulings
The mana value of a creature token is 0, unless that token is a copy of another creature, in which case it copies that creature's mana cost.
(wotc • 2012-10-01)
If the permanent has {X} in its mana cost, X is 0.
(wotc • 2012-10-01)
Rulings
The mana value of a creature token is 0, unless that token is a copy of another creature, in which case it copies that creature's mana cost.
(wotc • 2012-10-01)
If the permanent has {X} in its mana cost, X is 0.
(wotc • 2012-10-01)