What is a Coach worth?

Coach Kevin Rutherford and I have been musing on this subject: what meaningful measures can we propose to clients to evaluate the results of our work?

Kevin's made a start in thinking about this...
silk and spinach: process improvement metrics - some questions
Hopefully we can discuss this with other coaches at Agile2006 in OpenSpace.

Coaches: by what measures do you evaluate your work? It's tough, isn't it? What are your frustrations?

Further reading:
Value-Based Fees : How to Charge—and Get—What You're Worth
by Alan Weiss (on Amazon)
(caveat: I've not read this yet, but it's well reviewed)