It's worth whatever the market will bear. And most of his usual target audiences have deep corporate pockets.
That CNN story almost certainly vastly underestimates Greenspan's income on the lucrative speech circuit since "retiring" one year ago. The top earners' honoraria are highly variable, not fixed, from one occasion to the next.
As previously mentioned, it's been reliably estimated by multiple sources over the last year that it took him only about 6 weeks (rather than a year) to exceed his earnings during his entire 18-year tenure at the helm of the Fed.