Thanks very much for that link to the lensrentals.com article. That was really interesting reading, and a unique experiment, because who else could have all those filters available for the study? One thing to learn from all the feedback on this subject; opinions vary.

A long time ago, in the time before computers, my Nikkormat swung and bumped a rock on a trail in Hawaii (Kauai, where the road ends), and I was dumbfounded to see the filter all dented and smashed, broken glass all over, and the lens not affected. That taught me to not bump rocks. Don't remember exactly, but I probably went without a filter after that (since my filter was broke).

As far as paying more for the best filters, some things just have to be taken on faith, and not that it matters, but I think there's a whole lot of ways of melting stuff down and calling it "glass".