The hard part is the design. I have to hire someone. I don't really know how much I'm supposed to pay them but I paid my designer 50$ for that one. That gets costly when it's just a one off. And I don't even count the time I had to put in myself to come up with the general idea then discuss a bit with designer. Costly, timely and kind of a sucky process for me to go through.
I wish I could just take book orders like that but I would have to ask for too much money I think. I could probably let myself tempted for a decent offer tho. But in general of people don't come at me with their budget and a decent one I'm going to say no for now. It makes more sense to work on books I might be able to sell in large quantities.