After the initial excitement ;-) had settled down in the mid-nineties, it was, on order of magnitude, about $100 per year. Every once in a while, an interested mathematician would buy a copy. Other than that, in order to sell such a book, there has to be a graduate level class on the subject at some university (attended typically by about 5 people), and the teacher has to pick your book over a handful of other options.

I haven't kept track of the grand total that I made off the book, but it would certainly be a rather crappy used VW Golf.

I am so far removed from academia now that I have never thought about how I would publish a book like that today. My first impulse would be to make it a free ebook. Web search for "free course textbooks" indicates that this is not unheard of. Does anyone know how common it is, at various levels of higher education?