"As of January 2022, the platform had approximately 40 million monthly active users."[0]
In 2022 they had $30M expenses, so the cost is somewhat under $1/user/year.
They said the minimum donation is there to reduce the viability of scammers using it to check if a stolen credit card number is valid.
They are paying their CEO $0. Brian Acton, previously founder of WhatsApp, the guy who initially bankrolled the whole non-profit by a $50 million, 50 year, non-secured, 0% interst loan, later giving even more.
Source: https://en.wikipedia.org/wiki/Signal_Foundation#History
They pay some other C levels 400-600k. Source: https://projects.propublica.org/nonprofits/organizations/824506840 (compensation section)