No, the low cost doesn’t apply…

But solely as a result of Bontu’s Monument by no means applies to Costly Plunder. Bontu’s Monument applies to black creature spells, and Costly Plunder is an on the spot.

But, if it might apply, it will.

Consider Stormscape Familiar, which reads:

White spells and black spells you forged value 1 much less to forged.

If you forged Costly Plunder and sacrificed Stormscape Familiar to pay for it, you’ll solely must pay B, not 1B. This is as a result of the overall value you must pay for a spell (on this case, B and sacrificing a creature or artifact) is “locked in” earlier than you start the method of really paying for it.