Price.  Here is a biggie.  I have mentioned before that I see $0.025 and I see $0.33 CPMs.  I see $0.01 and I see $0.03 CPCs.  Atlas is notoriously the most expensive and Zedo and Mediaplex the least expensive.  Cheapest has its hidden costs in reliability of technology, lack of or quality of client support, inefficiency of scalable architecture, poor ease of use and a potential limitation in the number of sites that will accept the ad server’s tags (I haven’t touched that one before – but not all sites accept all ad servers).  The most expensive ad servers also does not mean they are the best. 

Prices are usually tiered, getting cheaper with more use.  The bands can run something like this:

• 1-25 million impressions/mos
• 25-50 million impressions/mos
• 50-75 million impressions/mos
• 75-100 million impressions/mos
• 100-300 million impressions/mos
• 300-500 million impressions/mos
• 500-1 billion impressions/mos
• >1 billion impressions/mos

Not everyone sees all of those tiers and every agency and advertiser will see a variation on these tiers and on the rates.  But that’s a pretty standard example.  Sharing with an ad server during the sales cycle your current impression volume is important.  And being accurate is also important because they will provide you with preferred rates.  But also be sure to share what you believe your growth will be as they will give you the higher tiers.  Don’t make the mistake of saying we get 50M impressions because you will only get tiered up to 50 or 100M impressions.  If you grow your rate will get stuck at that top tier and you could get better rates as you grow if you have the higher tiers in your contract.