The limit
Where mockups stop working.
The moment you put money behind a mockup, it has to do a different job. An advert has to make a stranger who has never heard of you want this specific story — a hook, a promise and a reason to click, pulled from what is actually in the book and aimed at readers who already buy that kind of book.
A mockup cannot do that, because it does not know what is in the book. It knows what the cover looks like. That is why a beautiful mockup can run for a week and return nothing: it is showing readers an object rather than telling them a story.
This is worth being blunt about, because plenty of authors run mockups as ads for months, conclude that ads do not work for their genre, and stop — when what did not work was showing strangers a photograph of an object.