What a stupid question. They make money off the sale of the game, the subscription fee (which is just a steady flow of free money that goes towards nothing) and expansion packs. The only cost in running a server is the front-end cost setting it up. After that you hire onstaff techs to maintain it (which is nothing) and pay the fee of electricity to run the servers and massive AC units for the server rooms. Part of my job is running a massive server for a biomedical research center. There is little to no maintanence cost. Atleast not one that would warrent 15 dollars a month from 10million people.