I've read multiple opinions on bandwidth vs. cost, but I think I could get away with about 1k concurrent online @ 1.5 mbps. I'm not sure if those costs you listed are reasonable though. You could run several virtual servers on one machine, and handle thousands of clients.

However, would I need to use linux? Is that even possible with 3dgs? I only ask because, an open source option may e the only economicaly viable solution if I have to pay for OS rights.