Dominaria Spoiler Digest - Who's Who and What's What from the Release Notes
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.)
3/14/2017 If you cast an instant or sorcery with in its mana cost this way, you still choose the value of X as part of casting the spell and pay that cost.
3/14/2017 If an instant or sorcery card in your graveyard already has flashback, you may use either flashback ability to cast it from your graveyard.
3/14/2017 You may pay any optional additional costs the spell has, such as conspire costs. You must pay any mandatory additional costs the spell has, such as that of Bone Splinters.
4/18/2017 For split cards, the flashback cost you pay is determined by the half you cast