Kellan, the Kid

If the spell you cast that caused Kellan, the Kid’s last ability to trigger has {X} in its mana cost, X is the value chosen as that spell was cast for the purpose of determining its mana value.

If you cast a spell “without paying its mana cost,” you can’t choose to cast it for any alternative costs. You can, however, pay additional costs, such as kicker costs. If the spell has any mandatory additional costs, those must be paid to cast the spell.