It sort of depends on the audience. I go to the SuperComputing conference every year, and there have been some excellent speakers. Michio Kaku was probably one of my favorite keynote speakers over the past 13 years. Alan Alda was also another great speaker. No idea what their costs are, but it's well worth it!

The reason that I say it "sort of" depends on the audience, Alan Alda was the Keynote at SC one year. You'd have to think an actor wouldn't relate well to a bunch of supercomputing dorks, but you'd be wrong. It was a very well received and rated talk. He's very good.