Dollars are pro rata: a plan’s price divided by its credits, times the book’s. Unused credits stay in your balance, and none of them expire.
Any paid plan includes the PDF, EPUB and DOCX of every book you write, for as long as you’re subscribed. A credit pack bought on its own adds credits for writing; bought from a book’s page, it also pays for that book’s files. The free credits write books you can read and edit in full in the app.
Extras cost more: web research adds 5 credits to a book, and regenerating a chapter costs 13 credits on a lead magnet.