Same per click, it obviously includes links when looking for products, but impression could also be counted and arguably especially if present in the first few sentences it is a valuable impression.

But then the separation of ads from content is lost so it becomes useless as product search, so maybe it isn't that trivial indeed. But it's not like even 10% of users is gonna find some other "search" engine and switch.

edit: can't reply deeper and interesting question, I mean we all would love to have ability to search arbitrary strings and regexes through the web corpus, but currently when you type something you get that AI reply instantly for most queries, this makes me still use them, if you forgot some shortcut key or something it has currently unique value in terms of latency (even ignoring the fact that for most users you also use them by default by typing in the address bar)