You must still pay the mana cost of the target instant in your graveyard to cast it using Toshiro’s ability.

Creatures destroyed by an instant that you cast normally can cause a trigger that allows the instant to be cast again. For example, cast Terror on an opponent’s creature. The creature is destroyed, triggering Toshiro’s ability, and then the Terror is put into your graveyard. You can put Toshiro’s ability on the stack targeting the Terror and then recast it when the ability resolves.