I've tested a lot of services on several different sites and Adsense has consistently performed the best.
I have a range of sites earnings anywhere from $0.50 CPM to $50 CPM using Adsense.
It really depends on the type of site you run. Sometimes it's not even worth placing ads if your traffic or quality of visitor is low. You may do better with lead generation.
A couple of other ad networks you may want to test are:
http://www.azoogleads.com - for affiliate/lead gen opportunities http://chitika.com - product based advertising
What kind of startup are you thinking about?
I've had some success buying display ads on specific sites through BuySellAds, just beware that all their biggest publishers are web design blogs that, for whatever reason, never convert sales for any product I've tried, even directly related to that profession. I guess the people that read "25 fresh green web designs" articles every morning have no disposable income.