How to request something in exchange for a conversation?

I text a handful of my subscribers frequently, especially one particular sub. And by frequently, I mean at least 20 messages every day. mostly sfw content, but occasionally he becomes lustful and leans somewhat toward nfsw content—not sexting. Instead, he’d prefer to do stuff like that with me. I try to avoid getting into it and instead attempt to respond politely by saying “thank you” and such things while trying to keep my responses general.

The problem is that he has never left me a tip or purchased anything. This has been going on for about a month. How would you respond in this circumstance? Or how would you (respectfully!) request a tip or something similar?