Why does one pay a relay at all? For read filtering and targeted feeds á la filter.nostr.wine. IMHO it should be a subscription by time.
Discussion
You pay so that others will not automatically assume your content is spam. Reading from a paid relay means you are confident that there is very little spam in the feed. Creators pay for that discoverability.
You can stick to 100% free use, but majority of users will filter you as spam unless they are your follower (or maybe your follower's follower).