There is a certain amount of sharing resources that seems to make running relays very cost efficient. For relay.tools there are 3 tiers of sharing. The server is shared by multiple relays, each relay is shared by multiple users. One popular pattern for sharing right now is someone runs a relay and adds their follow list (or any number of lists) to it's access control and/or enables payments to keep spam out.

This is good for the outbox model because clients will be more likely to hit one of these relays where most of their friends notes are without going overboard on connections.

Relays only get used if the relay operator promotes their use, otherwise they mostly just sit there idle (but usually do start receiving traffic from blastr, and scraped by nostr search engines when nostr watch discovers them).

As nostr moves to more of an outbox model and/or nip42 auth it will become clearer the cost per user, as it is now it's fairly nebulous. That being said strfry is very efficient and can easily handle 10k simultaneous connections on a low tier VPS.