How about a building server exclusive to players who have donated?
That would probably drive the cost up more, unless Ephialtes has a spare computer that he wouldn't mind constantly running a dedicated server on. He would have to pay to host the building server as well as the RTB servers.
I'm quite sure that's what Badspot does with his Freebuild server.
Editing to comment on the topic of the thread...Edit:
I like the idea. I wouldn't be very happy if I had to pay $100+/month for a server. It's only fair that we (the users) have the opportunity to help you pick up the bill for the service we take for granted.
I like how you're keeping it a free service too, I don't (and I hope you don't) want to see RTB become a pay-to-use service for everyone.