Lots of people focus on CPC (cost per click) but ultimately the performance measure that really matters is CPA (cost per aquistion). These two metrics are tied together by the conversion rate:CPA = (cost/click)/(conversions/click) = CPC/CR.
The chart below shows median CPC, median conversion rate, and median CPA during the 2007 holiday season for US Google AdWords advertisers who use conversion tracking. The series have been normalized so they all start at the same point on October 1. Click on graph below.
It is also of interest to look at how the number of clicks changes during the holiday season. Click on graph below