Ours starts to put them on sale about 10 days before Halloween. They start at 25% off and go to about 30% off by the day before. After Halloween they go to 50% off.
I highly doubt that the costumes will be going on sale this weekend. At my store, we don't even have all of the costumes in at this point.
My best advice is if you find a costume you like, buy when you see it because regardless if it is on sale or not, it may not be there the next time you are there. Granted, it may not fit in with the 'budget' theme of the thread, but at least you'll have the costume.
However, that being said, strangers things have happened so they may discount them, who knows.
I too have seen them discounted very early online, not in store, but just online. Not exactly sure when, but I know it's before Oct. I would keep checking, it will probably be soon.
costco has a few Princess costumes for $22.95. It's a dress, tiara & wand set by Disguise. Mine has Belle, Snow White and Tink. Nowhere near as many as they did last year.