I recently had the same experience. I rode in a dressage clinic with a grand prix level rider and learned more in 1 hour than I've learned from my last 5 instructors combined. It was very eye opening and left me convinced that good riders come from good instructors. Like you, this guy is way out of my price range for normal lessons, but I got so much out of it, that I am considering just doing one lesson a month with him instead of 1 a week from my average instructor. It is nice to have that big light bulb moment, isn't it?