just got a royalty statement for Your Linux Toolbox and apparently since it was published in 2019 I've made $14,707.16 in royalties, on 6,314 sales. So I guess around $2/sale on average.
Not mad about it at all (my goal was to learn about how traditional publishing works, and I did!) but those numbers definitely made me want to stick to self publishing, which pays a full-time salary for me, plus two employees part time, with some left over
(post when it came out: https://jvns.ca/blog/2019/10/21/print-collection-of-my-first-7-zines/)