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 {1} less.

A spell you cast that’s both creature types costs {1} less to cast, not {2} less.

When you cast an Elemental spell by paying its evoke cost, this effect reduces the cost to cast that spell by {1}.