Earlier quoted context omitted.
How much of a royalty are we talking about in say 2014? Enough for a cup of coffee or for a used VW Golf? I know that it is rare for modern programming books to make much money for their authors. Thus I imagine for a highly specialized math book it would be even rare.
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 option…
Free would be great! I think a lot of students go the alternative route and torrent books or just Google to find their respective .pdf's without paying after they see the price tag at the campus store/amazon/ebay.