There is a relay operator model where you pay per transaction ;) That's what Nostr Plebs will be implementing. Current registered ID holds will get XXX amount of notes/events before they have to start paying per transaction. We're still working on the details.
Discussion
How would this functionally work with a user's lightning wallet? Will they get an invoice every time they try to send a note to the relay? Or is it more frictionless than that somehow? I'm personally more in favor of the monthly subscription model but you have to pay more attention to resource management and adjust pricing.