Does kicker cost count as part of the spell's mana cost or is it an additional cost?

Kicker is an additional cost defined on a spell, separate from the card's printed mana cost. When you cast a spell with kicker, you may pay the kicker cost in addition to the mana cost (and any other costs). This is explicitly stated in CR 702.33a: kicker represents an optional additional cost.

Because kicker is an additional cost and not part of the mana cost, it does not affect the spell's converted mana cost (now called mana value). The mana value of a spell is determined solely by its mana cost as printed in the upper-right corner of the card, regardless of whether kicker was paid (CR 202.3).

This distinction matters in several practical ways: effects that reduce or pay a spell's mana cost (like cost-reduction abilities) do not automatically reduce the kicker cost. Likewise, if an opponent counters a kicked spell, they do not get the kicker benefit — the kicker only matters if the spell resolves.

Example: You cast Territorial Allosaurus (mana cost 2G) and pay its kicker cost of 3G. You spent 5G total, but the spell's mana value on the stack is still 3 (from its printed mana cost {2}{G}). An effect that says "counter target spell with mana value 5 or greater" could NOT counter it.

In summary: always treat kicker as an optional additional cost layered on top of the mana cost, with the mana value of the spell remaining unchanged whether or not kicker is paid (CR 702.33a, CR 202.3).