For a nostr client to cover expenses and salaries it really wouldn't take many people to subscribe. You need scale at the network level to make it happen but it's VERY doable - just a numbers game. 

Don't forget, not all subscribers are the same. Some are happy to support the products they use at different levels. We cannot assume Twitter paid subs are all paying the same price that would be insane. Companies paying to subscribe are likely accessing other services. Startup rule #1, scale pricing.