What happens if I pay and then someone takes my notes from other relays and throws them into this? That will exhaust my credits? The solution is to require AUTH for publishing such that only you can publish your own notes to this relay. That's built into NIP-70 which is enabled by default on khatru, you just have to require all events to have the "-" tag. Then the question becomes about how to properly publish to this relay, and then we will need some specialized clients or some special relay view section within clients with tools for publishing relay-specific notes.
Right now when I browse wss://ppe.swarmstr.com/ (by the way, it's just one click on the relay URL to browse on Gossip now) I see a bunch of notes that are replies to other notes that are outside the relay. But it would be cool to have something like a feed of notes that people paid to publish specifically to that relay, like a privileged billboard, I don't know. Maybe you could even reorder the feed such that highest paying notes are returned first instead of the most recent -- or ranked by a combination of newest+more expensive, I don't know.