Thanks! I've added a section at the bottom of the site showing some real photos of an actual coloring book I got in the mail. There are thumbnails of all photos uploaded on the back.

$24 + postage is the lowest I could reasonably charge for this. Printing costs are a bit more than half of that, OpenAI charge a surprising amount for image generation, but there is also a good amount of human effort (and creative choices) in generating the book. It's not a fully automated process and I hope that's evident from the quality of the end product.