Just into more data in the last 14 months. This is on a VERY high traffic publisher site; over 200 million page views a month.
Jan 2012 - December 2012
I looked at the CPC prices paid for first 5 days on each month. There was only 1 month in all of 2012 where the monthly CPC average was lower than the first 5 days. Put another way, earnings got better 11 out 12 months as the month went on.
2 months in a row the monthly average CPC was lower than the first 5 days. And the drop off in CPC prices compared to the first 5 days was significant (over 15% drop).
This is a trend that is developing; there is a major change going on with earnings from AdSense and the direction of CPC prices being paid out is not good.
Expect a record high Google earnings report at their 1st Quarter conference call.