I love it. And I think he's right: If attendees can't count up benefits gained to equal the cost of admission, why the heck would they come back? It would, at the very least, give attendees a reason to go back over those notes, sift through those business cards, act on that "aha" insight they had but forgot about by the end of the meeting two days later.
Actually, I've been to some meetings where, if this metric were in place, I'd end up paying 10 times the actual price. And others that wouldn't come close to breaking even. One thing is for sure, I'd be paying a lot more attention to everything if I were trying to quantify it.