The thing is ... ideas matter just as much as the writing. You cannot simply say: "Hey Claude, write me a Sherlock Holmes pastiche set in the bronze age."
I have found that a 1:4 to 1:8 ratio gives nice results. That is, for every 4-8 paras of expected output, you must provide 1 para worth of character detail, plot beats etc. So you would need 100-200 pages of your own writing to produce 800 pages worth.
But most people are lazy. So the output is too.