平和を勝ち取るために、これからどれだけの代償を払わなければならぬか、予想もつかぬ。

、、。

We can't even imagine how much we'll have to sacrifice to win peace.

Literal

Peace [object-を] win-take [for-ために], from-now how-much's cost [object-を] pay must [question-か], prediction [even-も] not-possible.

A powerful sentence with classical elements. 勝ち取る is 'to win/achieve (through struggle).' 代償を払う is 'to pay a price/sacrifice.' 予想もつかぬ uses the classical negative ぬ instead of ない — adding gravitas. The embedded question どれだけの代償を払わなければならぬか leaves the answer unknowable.