Since it doesn't say to pay "as an additional cost", you pay the life as it resolves (and if you entwined it, that would be after you gained the life from the sacrificed creatures).
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DCI Level 2 Judge
Scientists have calculated that the chance of anything so patently absurd actually existing are millions to one. But magicians have calculated that million-to-one chances crop up nine times out of ten.
If you choose the second mode of Plunge into Darkness, then when the spell resolves, you choose what X is when that spell resolves and pay that amount of life (C.R. 608.2c-d, 107.3e; choosing X as the spell is cast is not the "appropriate time" within the meaning of C.R. 107.3e, since the X here doesn't appear in a "variable cost that will be paid as it's being cast" under C.R. 601.2b); you don't do so "as an additional cost" of that spell, for example.
Scientists have calculated that the chance of anything so patently absurd actually existing are millions to one. But magicians have calculated that million-to-one chances crop up nine times out of ten.
EDIT (Jul. 4): Deleted last sentence.