Cheapest priced copy (Return to Ravnica): $0.35 at Card Kingdom, near mint, their price list as of .
Soul Tithe 
Enchantment — Aura · Uncommon · Return to Ravnica · #23
Enchant nonland permanent
At the beginning of the upkeep of enchanted permanent's controller, that player sacrifices it unless they pay {X}, where X is its mana value.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Return to Ravnica | 23 | Uncommon | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 1 October 2012The 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.
- 1 October 2012If the permanent has {X} in its mana cost, X is 0.