Bulk Up

To determine the total cost of a spell, start with the mana cost or alternative cost (such as a flashback cost) you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.

To double a creature's power, that creature gets +X/+0, where X is that creature's power when Bulk Up resolves.

You can cast a spell using flashback even if it was somehow put into your graveyard without having been cast.