It seems you haven't done the due diligence on what the parent meant :)

It's not about "constructing a prompt" in the sense of building the prompt string. That of course wouldn't be costly.

It is about reusing llm inference state already in GPU memory (for the older part of the prompt that remains the same) instead of rerunning the prompt and rebuilding those attention tensors from scratch.

I said "prompting with the entire context every time," I think it should be clear even to laypersons that the "prompting" cost refers to what the model provider charges you when you send them a prompt.