alt Hacker NewsWell, ethics is weaker, when the other is unethical in their own way.
400 euro for a book is hardly ethical. If you must charge that much for a copy you shouldn't have written it because you simply couldn't afford ro write it. Passin your poor business judgement onto others is not ethical.
Prices for scholarly books are not set by the author but by the publisher. The author will often send you a PDF for free if you email and ask nicely.