I don't understand what increases with usage? CPU? Memory? Disk space? At the scale of nostr, it all seems pretty easy to handle. 

And why have relay operators left? Solely due to hosting costs? Admin time overhead?

Are there any real surveys on the reasons relay operators exit nostr?

I run relays/servers for various networks and average around 500 Mbps on the connection. I have unlimited 1Gbps xfer for the cost of the rack. 

What would it take run a really busy nostr relay? What is a really busy nostr relay in terms of hardware and bandwidth utilization?