STYLETHREAD -- LET'S TALK SHOP!

I'm on the hunt for a dress to wear to BF's Christmas Party. The invitation says "casual to semi-formal" but apparently all the women wear cocktail-type dresses. So that is what I'm trying to find. The problem? I'm on a budget. Here's my criteria:

- knee-length or above the knee (preferably above-- I would like to wear it for New Year's too, which will probably entail going to some type of bar, and therefore I don't want to look too formal)

- I like designs that are simple but well designed. I like embellishments, but prefer that the fit of the dress is perfect, and I want it to be somewhat "unique."

- And the kicker... I want to spend about $80... $100 max.

I've gone to Forever 21, H&M and a series of other stores that consisted of everything at the mall. Booo.... Any ideas? Seen any good deals? I only have until November 24, but could always have something shipped overnight if need be. Thanks for the help; I'm so frustrated!!!