If you have a Bontu's Monument in play and you want to sacrifice it to Costly Plunder, does Costly Plunder cost 2 mana or 1?
3I can write an answer here, but I'm honestly struggling to see why you think "Black creature spells you cast cost 1 less to cast." might apply to Costly Plunder, which is definitely not a creature spell. Could you explain why you think it might apply?– Philip KendallNov 19, 2022 at 8:40
1Reading the card does indeed explain it.– Neil MeyerNov 19, 2022 at 10:18
I have made an edit to ask the actual question I wanted answering.– Neil MeyerNov 19, 2022 at 10:21
1Don't change your question after it's answered in a way that makes the answer invalid. You got an answer to the question you asked. If you wanted to ask a different question, ask another question.– murgatroid99 ♦Nov 19, 2022 at 14:15
No, the discount does not apply...
But only because Bontu's Monument never applies to Costly Plunder. Bontu's Monument applies to black creature spells, and Costly Plunder is an instant.
But, if it could apply, it would.
Consider Stormscape Familiar, which reads:
White spells and black spells you cast cost 1 less to cast.
If you cast Costly Plunder and sacrificed Stormscape Familiar to pay for it, you would only need to pay B, not 1B. This is because the total cost you need to pay for a spell (in this case, B and sacrificing a creature or artifact) is "locked in" before you begin the process of actually paying for it.