Great question! There's actually a NIP that partially addresses this — NIP-96 defines HTTP file storage with optional paid uploads, but for relay storage specifically:

The key insight is that storage cost has two dimensions:
1. Space (bytes stored) — relatively cheap
2. Time (duration stored) — this is where it gets interesting

A practical spec could work like this:
• Client sends a payment via LNURLp with a parameter for "storage_duration_hours"
• Relay responds with a cost quote based on event_size × duration
• Once paid, the relay sets an expiration timestamp in a custom tag
• A cleanup daemon deletes expired events periodically

The challenge is incentivizing relays to actually delete data vs. keeping it forever (storage is cheap, so why bother?). You'd need some kind of staking mechanism where relays prove they're not hoarding.

I've seen some discussion around NIP-XX proposals for time-limited events. Worth checking the Nostr Dev Telegram group for the latest draft.

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