Kopala, Warden of Waves has the following text: Spells your opponents cast that target a Merfolk you control cost (2) more to cast. Abilities your opponents activate that target a Merfolk you control cost (2) more to activate.
However, ingame the cost also applies to spells and abilities you cast on your own Merfolks. For example, casting a (1) mana instant targeting a Merfolk you control while Kopala is on the field costs (3) mana instead. Equipping a (1) equip cost equipment to your Merfolk also costs (3) mana instead, etc.
This is 100% reproducible.
However, ingame the cost also applies to spells and abilities you cast on your own Merfolks. For example, casting a (1) mana instant targeting a Merfolk you control while Kopala is on the field costs (3) mana instead. Equipping a (1) equip cost equipment to your Merfolk also costs (3) mana instead, etc.
This is 100% reproducible.