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Creatureless deck built around spells that work well with rebound, for play with Cast Through Time. Unfortunately, rebound resolves at the beginning of upkeep, so cancels don't work by this method. Question 1: do spells with (X) in their mana cost rebound as if they were cast with the same mana as the prior turn, or do you have to pay the mana over again, or do they rebound as if (X) = 0? Question 2: do flashbacked spells get rebound, since they are exiled after casting?
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http://gatherer.wizards.com/Pages/Card/Details.aspx?multiverseid=198301 thats the official rules and clarifications for cast through time. Didn't notice anything that mentioned X spells but it may still be worth a read.
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Even though you posted this a while ago and probably found your answers by now, the verbiage of Rebound states "...if you cast it from your hand." so this would prevent it from working in conjunction with a Flashback spell. As for X costs and rebound, from my understanding this is an ongoing point of contention. However the verbal breakdown that makes most sense to me (and substantially more fair, as much as it pains me to say it as I love Cast Through Time) is that Rebound doesn't make a copy of the spell, it casts it again. As you can only choose to cast it without paying it's mana cost and X is a dynamic modifier of total mana cost, X cannot be modified and is assumed to be 0. The lesson to be learned is that Magic is more of an exercise in language than strategy.