Based on my own ad rates from AdSense, the difference in CPM between May 2013 and May 2015 is actually up, although I am down about 10% based on May 2015 to May 2014.

This could be complete nonsense, but my gut feeling is that BA may be finding it more difficult to sell space directly because ad networks are getting better and better at targeting people themselves based on their browsing history and cookies. Either that, or BA is still selling premium inventory but - as it is now being shared out between more blogs - each individual blogger gets less.

As an example, I use a UK network to sell some of my space. In Q1 they did a very lucrative deal with the Royal Navy at a CPM of well over $10. I got a grand total of 746 impressions of that because it was spread across all of their network publishers.