TubeMogul did some research of late to look at how CPM rates vary on sites by ad platform and exchange and the results were pretty drastic with variations up to $6.10 in CPM rates. That's pretty large all things considered and gives a good reason to be on a demand-side platform (DSP, like TubeMogul) that is hooked into several exchanges at once. You could get your ads on the same site and save lots of cash.
TubeMogul ties into some big names in advertising including AdBrite, DoubleClick, Nexage and SpotXchange along with online platforms like Blip.tv, Brightcove, Viddler and YouTube. So they've got a pretty good pool of data to pull from. You can see a full list of their partners here.
They found that, for site using more than one platform, the majority of real-time bid pre-roll volume comes from a very few sources, 77.6% of auction volume is done by just 13.7% of publishers. In fact, the ComScore top 100 rated sites account for just 28.7% of volume and only 2.4% are using multiple platforms.
The major find is that there are fairly large differences in CPM rates for pre-roll across those multiple platforms, $6.10 for all sites overall while slightly smaller at $5 for comScore top 100 sites.
They were sure to state that there is no platform that consistently had higher rates and that price varied within and between platforms.
Basically what that seems to boil down to is that in order to maximize inventory revenue, publishers have found that utilizing multiple ad platforms works best. Identical sites can even have large variations in CPM and a DSP could actually pay off in the long run because it could net publishers the highest rates possible more often.
You can read the full, short report at TubeMogul.