So I'd assume 20 minutes of an expert's time is worth $500, right?<p>Edit:
Seriously, I'd charge $500 if the client picked the time and $100 if I could pick any period during the day, on short notice.<p>I'd actually charge more for a client-picked 20 minute period than I would for a one hour period because a 20 minute period would be very unlikely to repeat since 20 minutes isn't really enough to demonstrate great expertise.