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.

Only instant and sorcery cards in your graveyard when Past in Flames resolves will gain flashback. Instant and sorcery cards that are put into your graveyard later in the turn, including the resolving Past in Flames, won't gain flashback.

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.

If an instant or sorcery card in your graveyard already has flashback, you may use either flashback ability to cast it from your graveyard.

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.