I did a post back that gave some insight on which ad spots have the lowest CPM’s (and hence the least competition and most opportunity) so I figure I’d do it again now to help everyone involved end the year strong. So, here we go!

UK males – all ad sizes. Surprisingly, the UK male has the lowest CPM’s on desktop traffic. 160’s are lowest, followed by 728, then 300’s.
Canadian males – 160’s are lowest followed by 728’s.
Canadian females – 728×90’s

And it looks like across the board on mobile web, 160×600 and 728×90 have the lowest CPM’s. 

** 728×90 also includes 960×110 and the small ads (since 3 small ads = 1 banner).

Hope this helps!

Ben

11/18/2013 EDIT:

2013-11-18_1255

Here’s proof ladies and gents. 2 campaigns with identical targeting/CPM.
Even though Canada has 1/2 the volume of the US, because low CPM.. you’ll get 2x as much impressions in Canada for the same CPM vs. the US.