Spells you cast that have both types cost less to cast, not less.

Ballyrush Banneret's ability doesn't change the mana cost or mana value of any spell. It changes only the total cost you pay to cast Kithkin and Soldier spells.

Ballyrush Banneret's ability can't reduce the amount of colored mana you pay for a spell. It reduces only the generic mana component of that spell.