If we're discussing $200+/hr consultants, you're going to do yourself a disservice if you don't find some way of objectively measuring your contribution.
No one ever argues with increases to the bottom line, so start there.
Even then, unless the training is in some convenient sales/marketing field where the resulting conversions are easy to quantify, you may not be able to put a dollar amount on the long term value of the knowledge and insights your people gained from the training for a long time.
As an extreme example, consider the possibility of a one-day training course for software developers taught by a world class expert for $10K. Maybe one of your developers learned a new defensive programming technique five hours into that day, and that technique later prevented a bug that would have cost your business $1M to rectify. Obviously that training had a very high return on investment, but how is the trainer supposed to demonstrate that to his next client?
For the rest of the training market it would come down to a client requirement for the service, general qualification to do the job, and typical market price - existing budgets and product pricing kicks in at this point rather than ROI on specific individuals or teams. We see this in the dev contract market as well and that's what the article is about - commodity resource vs stars.
tl;dr if you can't demonstrate exceptional value, you will trend towards commodity.
There are also many ways that any organisation with more than one person contributing could collectively achieve good results, while any given individual personally made anything from a disproportionately positive to a negative contribution.
As I said, demonstrating your personal worth through a direct measurement of your contribution is not necessarily easy with team sizes greater than 1.