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 might.

Consider Stormscape Familiar, which reads:

White spells and black spells you solid price 1 much less to solid.

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