As you said, it's hard to calculate an exact number. But if you think your search results are only worth $1/month, that's up to you to determine. I know if I was an ad-broker or profiler, I'd pay more than $1/month/person as that's valuable information, in my opinion. And Kagi is worth much more than that to me. Proton theorizes:
If Google Search market share is also 90% in the US, that’s over 274 million people using Google, and the company earns $393 per year from each of them.