The answer is an ugly one: It varies.
Factors affecting CPM earnings include:
- The ad network
- The advertiser
- The fill rate
- The ad size
- The CTR
- The percentage of page views per visitor
- The locations of your sites visitors
The only way to tell is to test.
720x90 is a popular ad format.
I can't remember an advertiser telling me that I had to display more than one of their ad blocks.
The most common requirement is that all CPM ads be above the fold, but you're already planning for that.