How is that not as clear?
The real world money to purchase those token (and anything else) come from people. Those people do not have infinite resources to spend on the game.
Ergo, if you exhaust their real world resources, and then offer them the best item in the game for $1, they wouldn't take it.
At that point, you've got the value of the total in-game economy. Because anything virtual after that would be worthless.
(Subject to demand curve, etc. But in aggregate it boils down to the same point.)
(You might be able to make the case that speculators without interest in the game would come in at that point to buy items, but I'd question how many would take the risk after the playerbase was already financially drained)