They're growing, not thriving. Right now my revenue covers about half my cloud costs.
Doesn't matter as it's all about the experiment, but paid relays at scale will need to make money. Not a lot - mostly they will be a utility business so the time costs of management can be spread across thousands? tens thousands? users.
My view is that it will shake out to annual costs of between $2-$5 per user per year, maybe less if management time is minimal.
Always remember, it's relay owners who has to deal with DMCA issues. THAT could be the time suck that really pushes costs.