I am NOT too clued up about the USA scene.
However "Run of Network" banner campaigns should get you around $1 CPM.
Obviously if you can find a good sales house, who is willing to go out and represent your site to potential Agencies and Advertisers, then the figure could be a lot higher.
In the UK, high profile sites are struggling to get £2 - £3 cpm from advertisers, so they are ending up with lots of "Run of Network" stuff at £1 cpm.
bearing in mind most Ad Sales houses will charge the client double, and keep 50% for getting you paid advertising.
Do NOT rely on the site revenue, purely from Banner Advertising, those days are long gone and unlikely to return.
If you are getting 3-5 million visitors, then I am sure there are other ways to "milk them"