Discount On Wedding Dress

Just a quick opinion poll. I`ve run into a problem with my wedding dress. The store that I ordered it from ordered the wrong size of dress. Luckly though, the store has the same dress in stock but 2 sizes larger. So now I`m getting the dress that has been in the store since at least last August, when I went in to try on dresses.
I`ve told the manager of the store that I want a large discount off of this dress since it has been on the rack so long.

Now the question is what do you guys think is a fair discount??