When Snapcaster Mage enters the battlefield, target instant or sorcery card in your graveyard gains flashback until end of turn. The flashback cost is equal to its mana cost. (You may cast that card from your graveyard for its flashback cost. Then exile it.)
9/22/2011 If the targeted instant or sorcery card already has flashback, you may use either flashback ability to cast it from your graveyard.
10/1/2012 You can't pay other alternative costs of a card you're casting from your graveyard using flashback. For example, if the instant or sorcery has an overload cost, you can't pay that cost.
7/1/2013 For split cards, you pay only the cost for the half of the card you are casting. 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.