Dave started a thread named PPC Analytics is only wishful thinking that possibly may be the best thread ever started at our forums, ever. The discussion goes deep into the question of the true measurability of search ads, compared to other marketing initiatives. I am going to pull out a few quotes from the thread, but if you have time, you should read it - it isn't that long.
But let's say the average revenue was in the area of 10 million and the PPC contribution was about 1 million and seasonal spikes and drops can be as much as 5 million... The margin of error would be grater than the PPC contribution. A drop in 1 Million could be easily attributed to other factors like seasonality or competitive conditions.
You can not measure how many people would have made a purchase anyway with search the way you can with Banner advertising. Search analytics can not measure lift against a control group who does not see search ads during the same period of time. The ads are either there or not. Lift if any in sales could be easily attributed to other factors like seasonality or competitive conditions for the reasons above.
The argument is, if the lift in search can not be accurately measured because of this limitation. You could argue that it might be possible to drive more sales on the website if you do no search marketing at all and save the money to do something else like a TV commercial, print ad or something cool or sexy that can't be measured either. Or just save the money.
Check out the thread, MugShot is doing an excellent job with his replies. Forum discussion at Search Engine Roundtable Forums.