What is the Typical CPM Rate for Banner Ads?

A fellow AIMS member is doing some research and sends in this question about ad networks and CPM...

1. Who would you say are the leading banner ad networks to align oneself with?
2. what is the typical/avg cpm rate nowadays for banner ads…… ballpark figure is aok

A bit of background...

• Banners would be posted on a gaming site/portal i.e. fun games
• Users would be across various demographics and not targeted in any particular fashion
• What would be the cpm rate that the portal owner could expect from an ad network they partnered with

Anyone have any metrics or suggestions?