The best way to determine your price range is to look at five or six other speakers who are talking about a similar topic, and are on just about the same level as you, and then simply figure out a price that is about 85% of the average.
The topic was innovation, and featured speakers such as John Kao, the consultant and author, and Judy Estrin, scientist, entrepreneur, former CTO of Cisco and current board member at Disney and FedEx.