You know that it's very easy to get your book on Amazon through their subsidiary CreateSpace, right? You already have the PDF... Also getting it into the kindle store is free and easy. If you're just selling the ebook on your website, you're missing a trick.
I haven't really done any marketing except for the initial announcements. Most people find it directly on Amazon, or find the website by searching for "<language> book".
I self-published a programming book a while ago, and it brings in about $700 per month. I didn't see the big initial spike the OP did (shame), but the money has been consistent, and I would expect it to continue for several years.
Total start-up costs: $150 (proof copies etc.)
We're lucky as programmers that we tend to be able to wrangle LaTeX, and some of us have taste in typography and an ability to write decent prose. This means that the up-front cost of editors, typesetters, cover designers and so on is gone.
The disadvantages compared with my previous (real) publisher are a) no advance and b) no marketing and c) paying for translations to be done. Everything else has been an advantage.
When someone buys my book on Amazon for $35 I get $21. No traditional publisher can beat that.
Profit per copy $21/$9/$14 amazon/kindle/PDF
Haven't done any real price experiments yet.