I wonder what is the true server cost of one user to the system. Of course, the initial costs and human costs are quite high, but in a running system, what could be the cost?

They are E2E, so it is unlikely that the servers need to do any heavy processing. I see that I unused about 3GB of data for about a year, and with 3GB costing $0.30 even at AWS extortion prices, $1 seems like a good estimate.