> Put a different way, is there any device with a high monthly active user count that has a higher cost to purchase a black market exploit than the iPhone?

I'm going to answer about operating system rather than device.

The selling price of an Android full chain with persistence zero click is up to $2.5 million. The selling price of an iOS full chain with persistence zero click is up to $2 million.

https://zerodium.com/program.html

Both are better than any desktop operating system.