That is comparable to the fee the Google Display Network is charging [1], so that is not abnormal in that kind of business.

However, compared to Flattr and Brave it is still kind of high, which are charging around 10% [2].

[1] https://support.google.com/adsense/answer/180195?hl=en

[2] https://www.ctrl.blog/entry/flattr-vs-brave-fees.html