Bosk Banneret

• WWizards of the Coast2008-04-01
  Official Ruling
  A spell you cast that’s both creature types costs less to cast, not less.
• WWizards of the Coast2008-04-01
  Official Ruling
  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.

Rulings via Scryfall