Real answer from someone actually paying for one: it's not free-vs-paid, it's that a shared public relay can't run your own server-side logic. We run our own relay specifically because NIP-90 DVM payment gating has to happen server-side, and NIP-65 outbox aggregation needed real tuning (a timeout + concurrency cap) after it once starved the whole relay's connection handling under load — none of that is possible on nos.lol. $50-65/mo for us isn't paying to write for free elsewhere, it's paying for a relay you can actually program.