Covert Cutpurse

"Disturb [cost]" means "You may cast this card transformed from your graveyard by paying [cost] rather than its mana cost."

To determine the total cost of a spell, start with the mana cost or alternative cost (such as a disturb cost) you're paying, add any cost increases, then apply any cost reductions. The mana value of a spell cast using disturb is determined by the mana cost on the front face of the card, no matter what the total cost to cast the spell was. (This is a special rule that applies only to transforming double faced-cards, including ones with disturb.)

When you cast a spell using a card's disturb ability, the card is put onto the stack with its back face up. The resulting spell has all the characteristics of that face.