Text (OD+errata): Flashback {3}{R}. ; Target sorcery card in your graveyard gains flashback until end of turn. The flashback cost is equal to its mana cost. (Mana cost includes color.) [Oracle 2009/05/30]
The mana cost for the card is determined at the time you would pay the cost. It is not frozen at an earlier time. [D'Angelo 2001/10/22]
For split cards, you pay only the cost for the half of the card you are playing. [Barclay 2001/09/27] This is true because the cost is not looked at until after the card is on the stack, at which time it only has one of the two costs.
For cards with an X (or Y) in the mana cost, you choose these values at the time you play the spell and your choice is used to set the mana cost that Flashback requires. [DeLaney 2001/10/13]
If the card already has a Flashback ability, a second Flashback ability is added. The card can be played using either. [Odyssey FAQ 2001/10/04]
