I sell cheap but high-quality Anki decks for language learning: https://deckmill.com Created using a mix of automation (TTS, machine translation, etc.) and human reviews. Built it with a friend, making around $500 a month, very stable over the last couple of years. Spend 1 or 2 hours a month on it, mostly customer support.
LPT: use "affordable" instead of cheap, it basically means the same thing but has a totally different connotation