Varolz, the Scar-Striped

• WWizards of the Coast2013-04-15
  Official Ruling
  While Varolz, the Scar-Striped is on the battlefield, the scavenge cost of a creature card in your graveyard is equal to its mana cost, including any colored mana requirements.
• WWizards of the Coast2013-04-15
  Official Ruling
  If the creature card you scavenge has in its mana cost, X is 0.
• WWizards of the Coast2013-04-15
  Official Ruling
  Abilities that reduce the cost to cast a creature spell or alternative costs you can pay rather than a card's mana cost don't apply to activating a card's scavenge ability.
• WWizards of the Coast2013-04-15
  Official Ruling
  If a creature card has multiple instances of scavenge, you can activate either ability (but not both, as the card will be exiled when you activate one of them). Varolz's ability doesn't affect the scavenge cost of a creature's other scavenge ability.
• WWizards of the Coast2013-04-15
  Official Ruling
  Exiling the creature card with scavenge is part of the cost of activating the scavenge ability. Once the ability is activated and the cost is paid, it's too late to stop the ability from being activated by trying to remove the creature card from the graveyard.
• WWizards of the Coast2013-04-15
  Official Ruling
  The number of counters that a card's scavenge ability puts on a creature is based on the card's power as it last existed in the graveyard.

Rulings via Scryfall