Rule 702.1a

If an effect refers to a “[keyword ability] cost,” it refers only to the variable costs for that keyword. Example: Varolz, the Scar-Striped has an ability that says “Each creature card in your graveyard has scavenge. The scavenge cost is equal to its mana cost.” A creature card’s scavenge cost is an amount of mana equal to its mana cost, and the activation cost of the scavenge ability is that amount of mana plus “Exile this card from your graveyard.”