TypingMind.com is a "bring you own API key" (obviously, being a LLM frontend), that's also successfully monetizing users. The secret is that it's actually a very good product; until recently, it was far ahead of the official tools (I mean, they had plugins for like half a year before OpenAI started talking about "GPTs"), so paying for the license feels worth it (definitely was, when TypingMind was strictly better than ChatGPT Plus subscription).

It's also not a subscription - another reason "bring your own keys" apps are interesting, because you likely already have a paid subscription with the API vendor; adding another one on top of that needs some good justification.

Anyway, "bring your own key" users are a different market from general audience, and unlike the latter, it isn't already saturated with fly-by-night garbage and scam extensions, so you can both charge more for that feature, and have smaller costs marketing it.