Where can i find Highest CPM?

hi, i found that phpBB offer a $2 for 1000 impression advertise on their website.

http://www.phpbb.com/about/advertise/?s ... d35b25540a

And i found some also selling $0.5/1000 impressions.

So can any guru tell me where can i find the market rate for CPM?

Market Rate:
- CPL > unknown
- CPC > 0.25 usd
- CPM > ?

I am doing a research. Thanks