>>mikece+(OP)
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.
>>heavys+Cz
Phone numbers can be obtained anonymously in many countries. I have several anonymous Signal accounts, each with their own anonymous phone number.