Google Shopping Europe (GSE) has repeatedly confirmed that they keep a portion of all CPCs. However, GSE does not publish the exact size of this part. Therefore, no one knows exactly what percentage of CPCs are withheld by GSE.
At the beginning of the CSS program, some CSS providers ran A/B tests in which they pitted identical campaigns against each other with and without CSS. Several vendors have independently come to the conclusion that they had to reduce the CPCs in the CSS campaign by around 20% in order for it to get the same amount of traffic as the non-CSS campaign. For this reason, most CSS providers write that GSE retains 20% of the CPCs.
This means that at GSE a €1 CPC bid is reduced to 80 cents. Or to put it the other way around: to get an effective bid of €1, I have to enter €1.25. So 25% more.
That’s why at adstrong we say that a CSS increases effective CPCs by up to 25%.