Nostr relays are highly decentralized because we have negentropy and outbox models. That means its easier to cover the running costs, for every relay that isn't especially large and overrun with users.

You can't actually see most relays, as they're just little apps running on people's cell phones or laptops. I run the #Citrine 🍋 relay on my Android phone, and it has hundreds of books on it, takes up only 630 MB of space and doesn't noticeable increase my mobile bandwidth usage, since I sync it when in WiFi. Probably costs me a few euro-cents per month, and contains the notes that are relevant to me.