This puts the cost on the reader and doesn't correctly attribute the cost of load for a given pubkey on the creator of those events.
The main cost here is serving events so if one user is very popular then they will bring a lot of load to your relay
I think storage is an OK metric for now since it easy to keep the cost on the creator, if they create a lot of content and a lot of people are looking for their notes then they will pay more, the reader on the other hand pays nothing