I noticed that my Lord Zankai has a Volcano Gem that costs 28 mana, a chaos creature with a chaos gem. But in battle, it costs 42 mana. I wondered why this happened, then I realized that my class changes because of my Sphinx.
I don’t remember my spells always getting more expensive when a class changes, but then I remembered I had the perk “Spell Mastery” that allows my creatures to cast any spell, but it’s an increased mana cost if it’s not of their native class.
I think that the game is registering them as equipping a non-class gem, thus increasing their cost, even though they actually are that class but it’s just changed in battle.
EDIT: Yep, when Sphinx chooses Chaos, it’s normal cost.