Actually, traditional banner and CPC advertising is a very poor income source for games because of their low low clickthrough rate.
Basically, Google's AdSense will pay you 5c per click usually. But nobody will click your ads, because they'll be playing the game.
You may also think you can do CPM ads, where you get money per number of impressions or pageviews. However, revenue for this is also massively low and you can expect to get around $10 a month if you can get 200,000 pageviews in that time!
The best bet is CPA advertising - games really do live off of this from of advertising, and if you position it right to your players this will make you some real cash! In the first month of beta gameplay, buskerwars has made about $40 through CPA advertising from around 15 players.
I think I've made a thread about this somewhere
