Str82u - 6:08 pm on Jan 25, 2013 (gmt 0)
I tend to think of it the other way around, when CTR is high, the cost of clicks goes down. I firmly believe (conspiracy theory) that advertisers not only automate their Adwords but know in advance certain days when traffic and CTR is going to be high and adjust their bids to that....
This seems to be the trend. When CPC is high, CTR is low.
Non conspiracy theory is simpler; because there are more click troughs it lowers the cost per click because the are more clicks to go around. It happens here nearly every time the CTR goes high on a site the EPC/EPC is about 20-25% lower; very rarely do EPC/CPC stay up during high CTR periods except holidays.