You really need to look at a longer term than “always profitable”. The suggested $5/mo number is low enough that most ppl will not think twice. What you need to do is model the system and understand: 1) that most ppl will never use $5 of resources; and 2) where the crossover to profitable point is. Then plan accordingly. It is going to be hard to it with no risk for some period.

OTOH, the relay is pretty lightweight on a per event basis versus a “unit of server”.