A number of writers are doing quite well selling short erotic stories for between 99 cents and $2.99. The latter may seem outrageous if you think of it as the price of a short story. It's less so if you think of it as the price of an orgasm.
The whole article is a really interesting read, not least because it explains the subtle dance authors have to carry out when putting up their self-published porn in order to ensure that interested readers can find something that matches their precise tastes without using any forbidden terms on the cover material:
Marketing on Amazon is done largely by inputting keywords when uploading your book. Keywords and phrases are search terms readers use. For instance, "gay young adult novel" or "strong female characters" or "zombie steampunk." In erotica, you can use the real terms in keywords even if they're banned from blurbs. So if you go to Amazon and type in the banned word "orgy," you'll get books that used that as a keyword but have discreet titles like The Arrangement. (Or less discreet titles that at least don't include "orgy.")
Amazon is aware of this, of course. It seems that they're less interested in outright banning all erotica than in banning certain types and in keeping a virtual brown paper wrapper over graphic language visible in the storefront.
Call me as out of touch as a high court judge, but I had no idea dinosaur erotica was a thing until I read about it in a Guardian books post and now I know, I'm not sure whether it is something I wish my wife or my servants to read or not...