From a number's perspective, I think you can look at anything between $0 - $3000 a month for a popular technical book on Amazon. Maybe there are some even up at $3000 - $5000. That doesn't include what you get from publishing it through other channels as well.
That's only on point of viewing it though. I like more to see success from the recommendation/readers ratio perspective: How many of the people that have read your book would recommend it to someone else. That's where word of mouth comes into play that I find so much more worth than any money. It guarantees your book's success over the long term, not only for the launch. As every startup launch: it's a marathon and not a sprint! So make sure you deliver high quality or iterate towards high quality.