Signal is an incredibly lightweight design. If all users donated $0.60/year, or 5¢/month, that would sustain operations and overhead.
Compared to most other services, that's an insanely low cost per user.
EDIT: It's actually way less than even what I remembered - they only need about $0.60/user/year in order to break even, not $3/user/month.
Apparently so, since Signal broke even last year.
(and what's more, I disagree with your premise. I think I'd everyone donated, the cost would be very spread out and it would be shockingly affordable, especially relative to the benefits we get)
That's way, way lower than most other services, including most messaging services.
And donations aren't Signal's only source of revenue. Over a third comes from other sources.
> So Signal probably needs something like $30-40/month from people who actually donate, which seems unlikely.
Well, they broke even last year, so apparently it does work out.
Price points are location-dependent, which is why it's a nice round figure for you in GBP even though Signal is not based in the UK.
Also, as you noted, they accept donations outside the app.
Running chat apps doesn't have to be super expensive if you know what you're doing.