From previous posts certain buildings further reduce the cost to hire champions. This is a known bug. If you want to know the work around, check your buildings that give the discounts and destroy them until it gets to 0.

The adventurers' guild, for one.

Try saving, then deleting your buildings slowly and see if that helps, then whenever you find the right building, load the game and just demolish that building.

I have no idea myself, I never pick natural leader without picking cruel, just because of this huge bug.
I still thinks its silly that if you do not have a weakness, pick cruel and natural leader for a combined of +1 attack and -50% cost of hiring heroes.. Bad design IMO

Sincerely
~ Kongdej