Barring an understanding partner/very good friend who is a good writer who will put maybe a few dozen hours into at least copy editing, you're very easily looking at breakeven at best for many books. (Cover design is arguably less important if you won't be in bookstores.)

If a book is supporting some other activity, it can still absolutely make a lot of sense. It's tough to make a lot of money directly but it probably won't cost you a lot either unless it involves traveling for research etc.

If you enjoy writing a book why would you value your time?