skibum - 7:18 am on Dec 11, 2012 (gmt 0)
Retargeting kinda works but you generally have to come up with some really creative measurements to show that it makes financial sense. Generally when measuring a search campaign, users who click and buy are counted in the ROI calculation.
With retargeting, sales from users who "see" an ad and buy at some point after "seeing" an ad are counted as sales revenue attributable to the ad campaign. They are also referred to as view through conversions.
If the conversions from the ad campaign and the search campaign are added together sometimes the total sales from the search and ad (banner) campaigns are greater than the sales for the entire site.
There is some fuzzy math that goes on there.