According to Pubmatic which benchmarks effective cost per thousand impressions (eCPM) data from more than 3,000 websites on a monthly basis, eCPM averages for March 2008 were $1.18 for small websites, $0.34 for medium websites and $0.38 for large websites.
Small Web site segment: Less than 1 million page views per month.
Medium Web site segment: Between 1 million and 100 million page views per month.
Large Web site segment: Over 100 million page views per month. (Via: Alootechie)