supposing it takes you N hours to write it and you value your time K dollars/hour than the cost is N*K that you may have spent doing something else be it consulting, writing technical articles for a magazine or spending time with your family.
Given the goal (3K$) I'd be inclined to consider this a "if I pre-sell this much, it's worth my time, otherwise I'll do something else".
I asked the author this very question and his response seemed to be that he has already been planning on writing the book for a long time and the goal is just a small compensation for doing something he loves.
As you said, $3k doesn't go very far if you're thinking in terms of $/hr.
The only issue for me with backing this project is, the author does not have experience being a write/teacher. As a developer he has experience documenting the systems he creates, but I'm not sure how easily that translates to writing a book aimed at teaching a complex concept like dataflow systems.
I'm the author of the book and it is true that this would be my first published book.
My life has been very busy up to just recently. Now that I have extra time I want to spend it productively. $3000 is the minimum I was willing to make to spend my time to create a complete book, not just a series of blog posts. In short, I'm home with the kids while my wife works.
I started "Relay" more than a year ago just as a personal project. But in talking to others about it, I found that many people have lots of questions about dataflow. At first I thought I would just create a short blog post about dataflow to attempt to answer the questions. So, I started working on it... and it kept growing... and growing.
Even topics that you are an expert in, when you look hard you can find new things to learn. I started collecting links to articles related to dataflow just for my own edification. Eventually I posted them all to a new subreddit /r/DataFlowProgramming so that everyone could benefit.
So... I had a lot of text already written, a huge collection of notes, and time on my hand and enough material for a book that I felt would answer many questions people have. I thought I would give it a shot.
Given the goal (3K$) I'd be inclined to consider this a "if I pre-sell this much, it's worth my time, otherwise I'll do something else".