That's really interesting, because I feel almost the opposite way. Estimating P(A) is easy, there are many factors I can look at, and I have experience with it. However, I don't have a lot of experience with people buying me drinks for various reasons, so I'm not sure how to update on that just from experience.

It's actually quiet possible that the magnitude of error I have in my estimations/assumptions renders them useless, but, even then, the exercise overall is pretty helpful to understand Bayes' theorem better.