So far, #nostr is quite small, about 10K daily active users according to https://stats.nostr.band/ , 66M notes according to https://primal.net/explore . This is not costing big bucks yet, although it is getting beyond peanuts for sure.

I agree with you, paying for infra will need to be figured out. I think that broader adoption will massively increase spam too, which is costly to combat. The great thing about the permissionless protocol is that literally anyone is free try out a new business model, and we'll see what sticks.