I assume you mean AdSense ... Have you tried to optimize your layout to increase your CTR ? You mention page impressions, do you have stats for unique visitors ? Traffic sources breakdowns, how much is organic ? What type of site do you have ? Forum / Blog / info site ?
CPM is very niche related, what is your niche (if you don't mind) ? Have you been approached by companies to display ads before ? Maybe you could try and sell some ads directly yourself to see how that goes ?
AdBrite has
CPM :
AdBrite - How AdBrite Works