Is there a way to amortize that cost over several queries, i.e. "pre-bake" a document into a context persisted in some form to allow cheaper follow-up queries about it?
It reduces prompt costs by half for those shared prefix tokens, but you have to pay $4.50/million tokens/hour to keep that cache warm - so probably not a useful optimization for most lower traffic applications.
That's on a model with $3.5/1M input token cost, so half price on cached prefix tokens for $4.5/1M/hour breaks even at a little over 2.5 requests/hour using the cached prefix.
Tediously, it would be possible to do this chapter by chapter in order to exceed the output limit building something for future inputs.
Of course, the summary might not fulfill the same functionality as the original source document. YMMV