Paid relays are not a sustainable model. If you pay a one time fee there is no incentive to keep the relays up and highly available for the ones hosting. Along with storing historical notes long term. With more users comes more overhead costs. So even if hadlve the same number of signups a month the usage could be exponential month on month. Subscription is another option but not user friendly for monthly usage seems.

Always interested in new approaches to relays, Spam filtering, and keeping at least base relaying free. This is essential to onboarding new users that may not be Bitcoiners.