Cost per wear! It's not that expensive if you'll wear it many times more than a non-awesome dress.
I call Phoebe baiting. :)But you do make a good point, especially when I am already sorely tempted.
Consider me baited. But I agree with Amber that you should go for the dress. I don't think, unfortunately, that you can do any more than guess ahead of time how much wear you'll get out of it. But I'd justify the dress as, it's fun to own gorgeous things, and that if you try to restrict clothing purchases to things you feel that kind of connection to, rather than a sort of eh, this looks nice and useful, then you end up spending reasonably. That is, unless you find that you love love love very expensive clothes (Chanel haute couture is, I'm guessing, pricier than that dress), or moderately expensive ones very often.