I'm running wss://nostr.oxtr.dev with a total monthly cost of only 40 EUR. It works fine but it's not the fastest one out there by any means. The nostr-rs-relay software is pretty CPU heavy. I could easily scale up to make it faster but that would probably put the monthly costs to something between 350-500 EUR a month (for a VPS with a dedicated vCPU) with >=32 vCPU cores. That's too much for me too keep up without charging people for it.

I'd imagine the Damus relay is pretty resource heavy and costly to run.