yeah. uh, so far I've made less than that on the book of xen[1] and the publisher has said that it is selling 'surprisingly well' - he (my publisher) warned me going in that writing technical books doesn't pay as well as most things a qualified person can do.
But then, I get a real kick when I walk into Fry's and see my book.
I dream of being a published author one day and walking into a book store and secretly signing my books :)
I have heard that technical books don't sell well, but then again, this author is not very well known and her books don't sell well either. She said that every agent and publisher takes on new authors hoping they become the next William Gibson (or other huge name). Most authors barely survive off of writing.
I co-wrote a book ('Web-Graphics for Non Designers') and later tracked it down in a Borders/similar store in London and got my photo taken with it there. Certainly a novel experience for anyone who's not a f/t author, but not so exciting that I'd be dreaming about it! I was writing to a very difficult deadline, fried my brain in the process and it was hardly lucrative.
From my experience, I'd be hard-pressed to be convinced to do it again. I got a few thousand as an advance for writing three of the books ten chapters and it involved research, a plan, fleshing out the content, providing supplementary assets and then a couple of rounds of editing. I've done better out of non-spammy MFA sites in terms of return on time invested.
But then, I get a real kick when I walk into Fry's and see my book.
[1]http://nostarch.com/xen.htm