Those $8.1 million did not come from Youtube advertisement: according to the original AP article, only $870,000 did.
https://www.google.com/hostednews/ap/article/ALeqM5jlcpd0qGl...
This estimate seems to be based on views for all of PSY's channel, i.e. 1.3 billion views. That works out as a $0.7 eCPM, which seems more credible than the $8 floated below in this thread.