"how does a cheap/basically free relay raise money..."

Free relays becoming excessively (expensively) large is counter-productive to the future of the protocol. Nostr's first strength is decentralization.

Free stuff tends to bad incentives and thus bad outcomes. Like incumbent centralized social media.

That said, there's a need for some free relays: allow people to play with the ecosystem, before they get serious and put some $ into what they value.

What does a sustainable middle ground look like, that works for both users AND relay operators? Very simple...

Read for free. Publish+store for a nominal cost. But provide something equivalent to a free trial.

Let the market take care of a) pricing and b) punishing relays that get stupid with ads and other weirdness.