Former RuneScape private server[0] hoster here. Our infra costs would be about 200$/mo, but for a high quality server like in the OP, if you ever wanted to compensate any developer a respectable amount for their work, you would instantly be in the millions. We had about 20,000 hours of professional dev work put in over 5 years for our game.
[0] https://2009scape.org