This is a nice theory, much promoted on HN by a few high profile posters a while back.
Unfortunately, it doesn't necessarily scale well beyond a team size of 1. This may or may not be a problem, depending on the kind of work you want to do.
It's also worth noting that if you're going to play that game, you need to be much more of an all-rounder than just someone with technical skills. This will suit some people very well, but it's not for everyone.
In short, demonstrating a direct return on investment is a safe and reliable way to convince clients of your worth if you have the option, but it isn't the only way.
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.
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.
Outside of really small organizations where you can have direct control of all parties, guaranteeing increased revenues (and/or profits) is pretty damn hard.
Guaranteeing lower costs, faster operations, increased efficiencies, etc., is easier to achieve, because it's largely within your scope (speaking from a software perspective).
But keep looking for the revenue lift opportunities. Even if you're kept from doing the "right thing", continue to be vocal about your position. If it has validity, eventually it's going to play out and at least you will have a "told you so" when your contract comes up for renewal.
Sometimes you win. Sometimes you lose. Sometimes you say "I told you so" and they say "fuck off" and you part ways.
You're number one mission whenever you get a contract is to understand the business and figure out how you can get them making more money.
That's the secret; you should always be framing your services as an investment. Demonstrate how you're going to raise their revenues (and do it!) so that it when it comes time for contract renewal, your worth is easily demonstrable (you paid X in exchange for Y millions).