The effect reduces the total cost of the spell, regardless of whether you chose to pay additional or alternative costs. For example, if you cast a Rogue spell by paying its prowl cost, Frogtosser Banneret causes that spell to cost less.

When you cast an Elemental spell by paying its evoke cost, this effect reduces the cost to cast that spell by .

A spell you cast that’s both creature types costs less to cast, not less.