Just be honest: "I'm glad to hear that things are starting to take off, but I've reached the limit of what I can invest as a volunteer on this project [mention

Just be honest: "I'm glad to hear that things are starting to take off, but I've reached the limit of what I can invest as a volunteer on this project [mention day job, family, neglected spouse or whatever as appropriate]. In order to justify going any deeper on this I think we'd need to start discussing a compensated role." BTW notice that you're phrasing the gap in terms of you, not them. That makes it a lot easier to parse (and respect), on their side.