There are sites that I put up their cpc and cpm ads, they are for game advertising. Maybe you could try, the rates are okay.
This one is cost per click:
http://www.game-advertising-online.com/This one is cost per 1000 impressions:
http://cpmstar.com/This one can be cpc or cpm:
http://intergi.com/I am not sure about the rates they charge since I am a publisher not an advertiser. They are good because their publishers are game sites or online entertainment sites. I don't recommend advertising with google because the bid is so high, each cpc might cost 50 cents it's too much imo.
I think a cpm model is better, if you are going to advertise your server.
I always suggest ppl to advertise not only on RO sites, because we have to open RO to new players. If we just pulling the same group of players here and there, RO will die because everyone is eventually going to leave and no new people join.