I'd much rather have a fixed monthy/annual with the chain they are part of included. I wouldn't want mentions that aren't part of a  chain I didn't participate in.
Per note is feels unpredictable, like if a note blows up, it could burn through my balance pretty quickly and then I'm on the hook for a much higher annual amount if I want to continue using the service. 

 The way Mazin is doing, you would only store your own events in that relay. The replies to your event must be saved somewhere else. 

But yeah, on "inbox" relays that will receive every reply to your note, I don't think a limit on the number of events makes any sense.  

 Yeah correct, thats the challenge with doing it as pay-per-use. 

This was intended more to solve the problem of “where do I send my 5 year old exit.pub tweets” then a personal inbox. 

I could make individual inbox relays that users rent monthly that would stream everything tagged by their pubkey from our aggregator but thats a separate product.