We estimate that by 2025, Signal will require approximately $50 million dollars a year to operate—and this is very lean compared to other popular messaging apps that don’t respect your privacy.
The load distributes across more shoulders automatically.
If you only host a server for yourself and 10 friends it costs next to nothing, if you have a big operation it can get just as expensive, it depends on what you are willing to do.
With centralized systems there is no choice but for the one centralized host to host everything.