The estimate does sound reasonable if it's an one-off payment. I agree that no one would pay that amount of money each year to keep adblocking from Firefox.
It's not impossible that people would pay Firefox that much yearly to keep their current user-base from using ad blockers. However, what is impossible is to imagine Firefox would have anything close to their current user base if people were prevented from using ad blockers. Most likely they would shrink to almost 0 users overnight if they did this. There are very few reasons to use Firefox over Chrome or Safari (or even Edge) other than the much better ad blocking (or any ad blocking, on mobile).
It's not impossible that people would pay Firefox that much yearly to keep their current user-base from using ad blockers. However, what is impossible is to imagine Firefox would have anything close to their current user base if people were prevented from using ad blockers. Most likely they would shrink to almost 0 users overnight if they did this. There are very few reasons to use Firefox over Chrome or Safari (or even Edge) other than the much better ad blocking (or any ad blocking, on mobile).