I just got back from taking NOTES at the Business Enterprise Institute annual conference. One of the keynote speakers was Ronald J. Baker who writes books on pricing. He has several books, all worth devouring, on how to set prices. Interestingly, he takes an economist's view as in the Karl Marx theory of time and labor.
You will be amazed how much thought and science goes into pricing everything that we buy. From airline tickets to cars to beer, very smart people are figuring out just how much of our money we are willing to part with.
He makes the strongest arguments I have ever heard against the billable hour rate as charged by lawyers, accountants and other professionals. He says that save Peter Drucker, there are very few management books that are respected as academic and well researched. Baker strives for this standard.
See CPA Dudes for a great example of value pricing in practice.
Combine Baker with Alan Weiss, and you have everything you need to know about charging fees. I will provide NOTES on Million Dollar Consulting after I re-read it.