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 pr…
Does it sell more as a PDF (via web site), Kindle, or paper book on Amazon? I've done the traditional publishing route before, too, and agree that the advance is nice but the percentage cut of sales is miserable.
Profit per copy $21/$9/$14 amazon/kindle/PDF
Haven't done any real price experiments yet.