What is the most expensive keyword?

Keyword costs vary according to factors such as the competition and popularity of the word. You can track keyword costs directly from Google Keywords Planner or keyword tools such as Ubbersuggest. But it would be best to use Google Keywords Planner. The income from these words goes to Google because it is a service offered by Google.
Google, on the other hand, pays the site owners, who allow these ads to run on adsense, according to certain metrics.