There are already several Nostr relays that require a one-time admission fee to use them. I am not convinced this is a winning payment model.
The cost of (in the case of a high-quality relay) maintenance, updating spam-filters, and generally keeping the relay always up and running will increase with more users. Especially considering things will go wrong and there will be some customer service involved as well. Guaranteed.
If what I've said seems correct to you, would you be willing to pay a recurring fee (in sats) to be able to use such a high-quality relay? Why or why not?
Bonus sats if you come up with a non-standard model that might work better.
Opportunity costs