It may be "nothing" special, but ignoring the people who have taken you to task for the amount itself, consider that writing programming books often delivers the biggest value in reputation and follow-on consulting jobs etc.
A lot of technical books (for that matter: most books, regardless of subject) never generate a living wage from the book itself.
A lot of technical books (for that matter: most books, regardless of subject) never generate a living wage from the book itself.