When a copy of a spell is put on the stack, by something like Reverberate the copy will be an exact one. If the original spell had any additional costs paid they will also be considered paid for the copy. Also, if the original spell had a variable cost, like a Fireball, the copy will also have the same value of X. These additional and variable costs will not be paid with the copy, you don’t need to do anything other than play and resolve the Reverberate.

For example, a Goblin has to be sacrificed as an additional cost to play Goblin Grenade. No other Goblins will have to be sacrificed in order for the second Goblin Grenade to deal five damage to a creature or player. This also works for optional additional costs like Kicker. If a kicked Burst Lightning is copied the copy will also deal four points of damage even though the additional four mana was not paid for the copy.