Googling around doesn't have a lot of info on the costs but two comments were:
"Maybe you need a more humble system to only serve 50k concurrent players and your tech is good enough to serve 10k people per box (that would be pretty elite). You end up with a one box shard and a game server that can expand to 5 shards, on demand.
So, if you use machines that normally go for $100 per month and get a bulk discount of even 50% , that would still be 5 x $50 servers or $250 per location with maybe 5 in the US and one in Europe at $1500 per month, for this example."
"500 concurrent users could probably be hosted (depending heavily on the nature of the gameplay of the game) at less than $1000 a month. I'm assuming a single one of your servers can run 50 users concurrently (optimized, and depending on the nature of the game, they could handle more), and so you'll need 10 servers running 24 horus a day, 30 days a month, at an Amazon.com-hosted price of $0.120 an hour"