All that said: roughly how many objects have you seen or otherwise had compelling experience of having been dropped in your lifetime, would you estimate? How many of those have fallen to the ground, and how many have floated to the ceiling? What happens to those numbers if we eliminating ones more theoretically likely to float than pens (e.g., helium balloons)?

At a guess, I'd say I've personally seen on the order of five thousand non-helium-balloon-like objects dropped in my life, and they've all fallen down. So just starting from there, I'd estimate a > 99.9998 chance that the next one will, too.

This a good point, 99.9% probably is too low, although I'd be a little worried to go as high as 99.9998%. I'm not sure how you got that figure anyway, if you've seen five-thousand that means Laplace's law suggests about 99.98% doesn't it? I've probably seen a similar number (maybe a bit less), and I can remember one that failed to fall downward (although it was in very high winds so perhaps I should have been able to predict it).

The other evidence probably doesn't count for a Bayes factor of as much as 100:1, since by that point a good part of the remaining probability mass is concentrated in hypotheses like "pens always fall downwards except under this one specific rare circumstance" and "I am completely insane".