It isn't. It's a normal sized sundae. It has lots of gold leaf in it. As mneme said, basically, you're eating money if you order it. We didn't order it.
It's served in a crystal goblet, which you get to keep. You get to eat it with an 18 carat gold spoon, I think with mother of pearl inlay, but you don't get to keep the spoon.
And, my thought at the price tag? "Hm, a thousand dollars? I can do two cons, easy, on that money. Okay, one if I treat Josh to a con and it's a WorldCon or World Fantasy level convention."
Note that this figure includes expenses from going hog wild in the dealers' room.