Meredith & Joshua say Signal costs $14m a year in hosting costs. Storage at $1.3m/yr, servers $2.9m, registration fees $6m, bandwidth $2.8m, and additional servers $0.7m = $14m. They say their total costs are projected to be $50m/year, soon.
I wonder Signal compares to, say, Whats App: another volume-focused messaging system that at least used to be run very lean.