Signal had 40 million active users in 2021 [1]. With 14 million in infra cost, that comes to .35 per user/year. Total expenses are about 33 million, so about .825 per user/year. All in all that seems very reasonable. [1] https://www.businessofapps.com/data/signal-statistics/
This is kind of the number I was looking for -- "Cover your own costs: $1/year. Cover yourself and five other people: $5/year." I feel like something pointing out that the costs are around $1/year on signing up, maybe with a reminder once a year, would get most people self-funding pretty quickly.
(Originally WhatsApp charged $1/year.)